diff --git a/toolchain/check/BUILD b/toolchain/check/BUILD
index 1eb437c1ff77..161b1ec462b1 100644
--- a/toolchain/check/BUILD
+++ b/toolchain/check/BUILD
@@ -102,6 +102,7 @@ cc_library(
     deps = [
         ":context",
         ":import",
+        ":interface",
         "//common:check",
         "//common:ostream",
         "//toolchain/base:pretty_stack_trace_function",
@@ -147,6 +148,20 @@ cc_library(
     ],
 )
 
+cc_library(
+    name = "interface",
+    srcs = ["interface.cpp"],
+    hdrs = ["interface.h"],
+    deps = [
+        ":context",
+        "//common:check",
+        "//toolchain/sem_ir:file",
+        "//toolchain/sem_ir:ids",
+        "//toolchain/sem_ir:inst",
+        "//toolchain/sem_ir:inst_kind",
+    ],
+)
+
 glob_sh_run(
     args = [
         "$(location //toolchain/driver:carbon)",
diff --git a/toolchain/check/context.cpp b/toolchain/check/context.cpp
index f55c1d8ff5df..7a8fdd82f4f4 100644
--- a/toolchain/check/context.cpp
+++ b/toolchain/check/context.cpp
@@ -173,7 +173,7 @@ auto Context::NoteUndefinedInterface(SemIR::InterfaceId interface_id,
                                      DiagnosticBuilder& builder) -> void {
   const auto& interface_info = interfaces().Get(interface_id);
   CARBON_CHECK(!interface_info.is_defined()) << "Interface is not incomplete";
-  if (interface_info.definition_id.is_valid()) {
+  if (interface_info.is_being_defined()) {
     CARBON_DIAGNOSTIC(InterfaceUndefinedWithinDefinition, Note,
                       "Interface is currently being defined.");
     builder.Note(interface_info.definition_id,
@@ -813,16 +813,13 @@ class TypeCompleter {
   // types, as found by AddNestedIncompleteTypes, are known to be complete.
   auto BuildValueRepr(SemIR::TypeId type_id, SemIR::Inst inst) const
       -> SemIR::ValueRepr {
-    // TODO: This can emit new SemIR instructions. Consider emitting them into a
-    // dedicated file-scope instruction block where possible, or somewhere else
-    // that better reflects the definition of the type, rather than wherever the
-    // type happens to first be required to be complete.
     switch (inst.kind()) {
       case SemIR::AddrOf::Kind:
       case SemIR::AddrPattern::Kind:
       case SemIR::ArrayIndex::Kind:
       case SemIR::ArrayInit::Kind:
       case SemIR::Assign::Kind:
+      case SemIR::AssociatedEntity::Kind:
       case SemIR::BaseDecl::Kind:
       case SemIR::BindAlias::Kind:
       case SemIR::BindName::Kind:
@@ -908,6 +905,7 @@ class TypeCompleter {
       case SemIR::Builtin::Kind:
         return BuildBuiltinValueRepr(type_id, inst.As<SemIR::Builtin>());
 
+      case SemIR::AssociatedEntityType::Kind:
       case SemIR::BindSymbolicName::Kind:
       case SemIR::PointerType::Kind:
       case SemIR::UnboundElementType::Kind:
@@ -980,6 +978,13 @@ auto Context::GetTupleType(llvm::ArrayRef<SemIR::TypeId> type_ids)
   return GetTypeImpl<SemIR::TupleType>(*this, type_blocks().Add(type_ids));
 }
 
+auto Context::GetAssociatedEntityType(SemIR::InterfaceId interface_id,
+                                      SemIR::TypeId entity_type_id)
+    -> SemIR::TypeId {
+  return GetTypeImpl<SemIR::AssociatedEntityType>(*this, interface_id,
+                                                  entity_type_id);
+}
+
 auto Context::GetBuiltinType(SemIR::BuiltinKind kind) -> SemIR::TypeId {
   CARBON_CHECK(kind != SemIR::BuiltinKind::Invalid);
   auto type_id = GetTypeIdForTypeConstant(
diff --git a/toolchain/check/context.h b/toolchain/check/context.h
index 134b5ff46ca1..b24759ee2bc6 100644
--- a/toolchain/check/context.h
+++ b/toolchain/check/context.h
@@ -234,6 +234,10 @@ class Context {
 
   // TODO: Consider moving these `Get*Type` functions to a separate class.
 
+  // Gets the type for the name of an associated entity.
+  auto GetAssociatedEntityType(SemIR::InterfaceId interface_id,
+                               SemIR::TypeId entity_type_id) -> SemIR::TypeId;
+
   // Gets a builtin type. The returned type will be complete.
   auto GetBuiltinType(SemIR::BuiltinKind kind) -> SemIR::TypeId;
 
diff --git a/toolchain/check/eval.cpp b/toolchain/check/eval.cpp
index d9f4722edd59..9ee1c427d5c6 100644
--- a/toolchain/check/eval.cpp
+++ b/toolchain/check/eval.cpp
@@ -322,6 +322,9 @@ auto TryEvalInst(Context& context, SemIR::InstId inst_id, SemIR::Inst inst)
             return true;
           },
           &SemIR::ArrayType::bound_id, &SemIR::ArrayType::element_type_id);
+    case SemIR::AssociatedEntityType::Kind:
+      return RebuildIfFieldsAreConstant(
+          context, inst, &SemIR::AssociatedEntityType::entity_type_id);
     case SemIR::BoundMethod::Kind:
       return RebuildIfFieldsAreConstant(context, inst,
                                         &SemIR::BoundMethod::object_id,
@@ -363,6 +366,7 @@ auto TryEvalInst(Context& context, SemIR::InstId inst_id, SemIR::Inst inst)
     case SemIR::TupleInit::Kind:
       return RebuildInitAsValue(context, inst, SemIR::TupleValue::Kind);
 
+    case SemIR::AssociatedEntity::Kind:
     case SemIR::Builtin::Kind:
       // Builtins are always template constants.
       return MakeConstantResult(context, inst, Phase::Template);
diff --git a/toolchain/check/handle_function.cpp b/toolchain/check/handle_function.cpp
index fa36daee5185..ce399b5e4c9e 100644
--- a/toolchain/check/handle_function.cpp
+++ b/toolchain/check/handle_function.cpp
@@ -5,6 +5,7 @@
 #include "toolchain/check/context.h"
 #include "toolchain/check/convert.h"
 #include "toolchain/check/decl_name_stack.h"
+#include "toolchain/check/interface.h"
 #include "toolchain/check/modifiers.h"
 #include "toolchain/parse/tree_node_location_translator.h"
 #include "toolchain/sem_ir/entry_point.h"
@@ -126,9 +127,24 @@ static auto BuildFunctionDecl(Context& context,
   auto function_decl_id =
       context.AddPlaceholderInst({parse_node, function_decl});
 
+  // At interface scope, a function declaration introduces an associated
+  // function.
+  auto lookup_result_id = function_decl_id;
+  if (name_context.enclosing_scope_id_for_new_inst().is_valid() &&
+      !name_context.has_qualifiers) {
+    auto scope_inst_id = context.name_scopes().GetInstIdIfValid(
+        name_context.enclosing_scope_id_for_new_inst());
+    if (auto interface_scope =
+            context.insts().TryGetAsIfValid<SemIR::InterfaceDecl>(
+                scope_inst_id)) {
+      lookup_result_id = BuildAssociatedEntity(
+          context, interface_scope->interface_id, function_decl_id);
+    }
+  }
+
   // Check whether this is a redeclaration.
   auto existing_id =
-      context.decl_name_stack().LookupOrAddName(name_context, function_decl_id);
+      context.decl_name_stack().LookupOrAddName(name_context, lookup_result_id);
   if (existing_id.is_valid()) {
     if (auto existing_function_decl =
             context.insts().Get(existing_id).TryAs<SemIR::FunctionDecl>()) {
@@ -136,6 +152,7 @@ static auto BuildFunctionDecl(Context& context,
       function_decl.function_id = existing_function_decl->function_id;
 
       // TODO: Check that the signature matches!
+      // TODO: Disallow redeclarations within classes?
 
       // Track the signature from the definition, so that IDs in the body match
       // IDs in the signature.
@@ -149,6 +166,8 @@ static auto BuildFunctionDecl(Context& context,
       }
     } else {
       // This is a redeclaration of something other than a function.
+      // This includes the case where an associated function redeclares another
+      // associated function.
       context.DiagnoseDuplicateName(function_decl_id, existing_id);
     }
   }
diff --git a/toolchain/check/handle_interface.cpp b/toolchain/check/handle_interface.cpp
index 8e643c9d4a7f..83005c24f925 100644
--- a/toolchain/check/handle_interface.cpp
+++ b/toolchain/check/handle_interface.cpp
@@ -130,7 +130,9 @@ auto HandleInterfaceDefinitionStart(
 
   context.inst_block_stack().Push();
   context.node_stack().Push(parse_node, interface_id);
-  // TODO: Perhaps use the args_type_info_stack for a witness table.
+
+  // We use the arg stack to build the witness table type.
+  context.args_type_info_stack().Push();
 
   // TODO: Handle the case where there's control flow in the interface body. For
   // example:
@@ -153,10 +155,13 @@ auto HandleInterfaceDefinition(Context& context,
   context.inst_block_stack().Pop();
   context.scope_stack().Pop();
   context.decl_name_stack().PopScope();
+  auto associated_entities_id = context.args_type_info_stack().Pop();
 
   // The interface type is now fully defined.
   auto& interface_info = context.interfaces().Get(interface_id);
-  interface_info.defined = true;
+  if (!interface_info.associated_entities_id.is_valid()) {
+    interface_info.associated_entities_id = associated_entities_id;
+  }
   return true;
 }
 
diff --git a/toolchain/check/import_ref.cpp b/toolchain/check/import_ref.cpp
index 75c676652848..75b1f9916e08 100644
--- a/toolchain/check/import_ref.cpp
+++ b/toolchain/check/import_ref.cpp
@@ -237,13 +237,9 @@ class ImportRefResolver {
   }
 
   // Adds ImportRefUnused entries for members of the imported scope, for name
-  // lookup. Returns the block used for the refs, primarily for textual IR
-  // formatting.
+  // lookup.
   auto AddNameScopeImportRefs(const SemIR::NameScope& import_scope,
-                              SemIR::NameScope& new_scope)
-      -> SemIR::InstBlockId {
-    // Push a block so that we can add scoped instructions to it.
-    context_.inst_block_stack().Push();
+                              SemIR::NameScope& new_scope) -> void {
     for (auto [entry_name_id, entry_inst_id] : import_scope.names) {
       auto ref_id = context_.AddPlaceholderInst(
           SemIR::ImportRefUnused{import_ir_id_, entry_inst_id});
@@ -251,7 +247,24 @@ class ImportRefResolver {
           new_scope.names.insert({GetLocalNameId(entry_name_id), ref_id})
               .second);
     }
-    return context_.inst_block_stack().Pop();
+  }
+
+  // Given a block ID for a list of associated entities of a witness, returns a
+  // version localized to the current IR.
+  auto AddAssociatedEntities(SemIR::InstBlockId associated_entities_id)
+      -> SemIR::InstBlockId {
+    if (associated_entities_id == SemIR::InstBlockId::Empty) {
+      return SemIR::InstBlockId::Empty;
+    }
+    auto associated_entities =
+        import_ir_.inst_blocks().Get(associated_entities_id);
+    llvm::SmallVector<SemIR::InstId> new_associated_entities;
+    new_associated_entities.reserve(associated_entities.size());
+    for (auto inst_id : associated_entities) {
+      new_associated_entities.push_back(context_.AddPlaceholderInst(
+          SemIR::ImportRefUnused{import_ir_id_, inst_id}));
+    }
+    return context_.inst_blocks().Add(new_associated_entities);
   }
 
   // Tries to resolve the InstId, returning a constant when ready, or Invalid if
@@ -411,7 +424,11 @@ class ImportRefResolver {
     auto& new_scope = context_.name_scopes().Get(new_class.scope_id);
     const auto& import_scope =
         import_ir_.name_scopes().Get(import_class.scope_id);
-    new_class.body_block_id = AddNameScopeImportRefs(import_scope, new_scope);
+
+    // Push a block so that we can add scoped instructions to it.
+    context_.inst_block_stack().Push();
+    AddNameScopeImportRefs(import_scope, new_scope);
+    new_class.body_block_id = context_.inst_block_stack().Pop();
 
     if (import_class.base_id.is_valid()) {
       new_class.base_id = base_const_id.inst_id();
@@ -591,12 +608,16 @@ class ImportRefResolver {
       auto& new_scope = context_.name_scopes().Get(new_interface.scope_id);
       const auto& import_scope =
           import_ir_.name_scopes().Get(import_interface.scope_id);
-      new_interface.body_block_id =
-          AddNameScopeImportRefs(import_scope, new_scope);
+
+      // Push a block so that we can add scoped instructions to it.
+      context_.inst_block_stack().Push();
+      AddNameScopeImportRefs(import_scope, new_scope);
+      new_interface.associated_entities_id =
+          AddAssociatedEntities(import_interface.associated_entities_id);
+      new_interface.body_block_id = context_.inst_block_stack().Pop();
+
       CARBON_CHECK(import_scope.extended_scopes.empty())
           << "Interfaces don't currently have extended scopes to support.";
-
-      new_interface.defined = true;
     }
 
     // Write the interface ID into the InterfaceDecl.
diff --git a/toolchain/check/interface.cpp b/toolchain/check/interface.cpp
new file mode 100644
index 000000000000..906ea8a100f5
--- /dev/null
+++ b/toolchain/check/interface.cpp
@@ -0,0 +1,36 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include "toolchain/check/interface.h"
+
+#include "toolchain/check/context.h"
+#include "toolchain/sem_ir/ids.h"
+#include "toolchain/sem_ir/typed_insts.h"
+
+namespace Carbon::Check {
+
+auto BuildAssociatedEntity(Context& context, SemIR::InterfaceId interface_id,
+                           SemIR::InstId decl_id) -> SemIR::InstId {
+  auto& interface_info = context.interfaces().Get(interface_id);
+  if (!interface_info.is_being_defined()) {
+    // This should only happen if the interface is erroneously defined more than
+    // once.
+    // TODO: Find a way to CHECK this.
+    return SemIR::InstId::BuiltinError;
+  }
+
+  // Register this declaration as declaring an associated entity.
+  auto index = SemIR::ElementIndex(
+      context.args_type_info_stack().PeekCurrentBlockContents().size());
+  context.args_type_info_stack().AddInstId(decl_id);
+
+  // Name lookup for the declaration's name should name the associated entity,
+  // not the declaration itself.
+  auto type_id = context.GetAssociatedEntityType(
+      interface_id, context.insts().Get(decl_id).type_id());
+  return context.AddInst({context.insts().GetParseNode(decl_id),
+                          SemIR::AssociatedEntity{type_id, index, decl_id}});
+}
+
+}  // namespace Carbon::Check
diff --git a/toolchain/check/interface.h b/toolchain/check/interface.h
new file mode 100644
index 000000000000..ae4b9dd1e244
--- /dev/null
+++ b/toolchain/check/interface.h
@@ -0,0 +1,22 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#ifndef CARBON_TOOLCHAIN_CHECK_INTERFACE_H_
+#define CARBON_TOOLCHAIN_CHECK_INTERFACE_H_
+
+#include "toolchain/check/context.h"
+#include "toolchain/sem_ir/ids.h"
+
+namespace Carbon::Check {
+
+// Builds and returns an associated entity for `interface_id` corresponding to
+// the declaration `decl_id`, which can be an associated function or an
+// associated constant. Registers the associated entity in the list for the
+// interface.
+auto BuildAssociatedEntity(Context& context, SemIR::InterfaceId interface_id,
+                           SemIR::InstId decl_id) -> SemIR::InstId;
+
+}  // namespace Carbon::Check
+
+#endif  // CARBON_TOOLCHAIN_CHECK_INTERFACE_H_
diff --git a/toolchain/check/testdata/impl/basic.carbon b/toolchain/check/testdata/impl/basic.carbon
index 3778fa4b9a7e..e7458d9429df 100644
--- a/toolchain/check/testdata/impl/basic.carbon
+++ b/toolchain/check/testdata/impl/basic.carbon
@@ -16,19 +16,23 @@ impl i32 as Simple {
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %.1: type = interface_type @Simple [template]
+// CHECK:STDOUT:   %.2: type = assoc_entity_type @Simple, <function> [template]
+// CHECK:STDOUT:   %.3: <associated <function> in Simple> = assoc_entity element0, @Simple.%F [template]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: file {
 // CHECK:STDOUT:   package: <namespace> = namespace {.Simple = %Simple.decl} [template]
 // CHECK:STDOUT:   %Simple.decl = interface_decl @Simple, () [template = constants.%.1]
-// CHECK:STDOUT:   impl_decl @impl, (<unexpected instref inst+4>)
+// CHECK:STDOUT:   impl_decl @impl, (<unexpected instref inst+7>)
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: interface @Simple {
 // CHECK:STDOUT:   %F: <function> = fn_decl @F.1 [template]
+// CHECK:STDOUT:   %.loc8: <associated <function> in Simple> = assoc_entity element0, %F [template = constants.%.3]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
-// CHECK:STDOUT:   .F = %F
+// CHECK:STDOUT:   .F = %.loc8
+// CHECK:STDOUT:   witness = (%F)
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl: i32 as Simple {
diff --git a/toolchain/check/testdata/impl/declaration.carbon b/toolchain/check/testdata/impl/declaration.carbon
index b5874fb0f0a9..ba72a4234234 100644
--- a/toolchain/check/testdata/impl/declaration.carbon
+++ b/toolchain/check/testdata/impl/declaration.carbon
@@ -23,6 +23,7 @@ impl i32 as I;
 // CHECK:STDOUT: interface @I {
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
+// CHECK:STDOUT:   witness = ()
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl: i32 as I;
diff --git a/toolchain/check/testdata/impl/empty.carbon b/toolchain/check/testdata/impl/empty.carbon
index 8bc787c98cf5..f0f588c13482 100644
--- a/toolchain/check/testdata/impl/empty.carbon
+++ b/toolchain/check/testdata/impl/empty.carbon
@@ -25,6 +25,7 @@ impl i32 as Empty {
 // CHECK:STDOUT: interface @Empty {
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
+// CHECK:STDOUT:   witness = ()
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl: i32 as Empty {
diff --git a/toolchain/check/testdata/impl/fail_extend_impl_forall.carbon b/toolchain/check/testdata/impl/fail_extend_impl_forall.carbon
index 25efc3bfd7dd..050c5e784992 100644
--- a/toolchain/check/testdata/impl/fail_extend_impl_forall.carbon
+++ b/toolchain/check/testdata/impl/fail_extend_impl_forall.carbon
@@ -27,8 +27,10 @@ class C {
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %.1: type = interface_type @GenericInterface [template]
+// CHECK:STDOUT:   %.2: type = assoc_entity_type @GenericInterface, <function> [template]
+// CHECK:STDOUT:   %.3: <associated <function> in GenericInterface> = assoc_entity element0, @GenericInterface.%F [template]
 // CHECK:STDOUT:   %C: type = class_type @C [template]
-// CHECK:STDOUT:   %.2: type = struct_type {} [template]
+// CHECK:STDOUT:   %.4: type = struct_type {} [template]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: file {
@@ -39,9 +41,11 @@ class C {
 // CHECK:STDOUT:
 // CHECK:STDOUT: interface @GenericInterface {
 // CHECK:STDOUT:   %F: <function> = fn_decl @F.1 [template]
+// CHECK:STDOUT:   %.loc11: <associated <function> in GenericInterface> = assoc_entity element0, %F [template = constants.%.3]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
-// CHECK:STDOUT:   .F = %F
+// CHECK:STDOUT:   .F = %.loc11
+// CHECK:STDOUT:   witness = (%F)
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl: C as <error> {
@@ -52,7 +56,7 @@ class C {
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @C {
-// CHECK:STDOUT:   impl_decl @impl, (<unexpected instref inst+11>, <unexpected instref inst+12>, <unexpected instref inst+13>, <unexpected instref inst+14>)
+// CHECK:STDOUT:   impl_decl @impl, (<unexpected instref inst+14>, <unexpected instref inst+15>, <unexpected instref inst+16>, <unexpected instref inst+17>)
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   has_error
diff --git a/toolchain/check/testdata/impl/fail_extend_impl_scope.carbon b/toolchain/check/testdata/impl/fail_extend_impl_scope.carbon
index 7c569c519941..fa3ece87cd93 100644
--- a/toolchain/check/testdata/impl/fail_extend_impl_scope.carbon
+++ b/toolchain/check/testdata/impl/fail_extend_impl_scope.carbon
@@ -26,6 +26,7 @@ extend impl i32 as I {}
 // CHECK:STDOUT: interface @I {
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
+// CHECK:STDOUT:   witness = ()
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl: i32 as I {
diff --git a/toolchain/check/testdata/impl/fail_extend_impl_type_as.carbon b/toolchain/check/testdata/impl/fail_extend_impl_type_as.carbon
index 2516664a6642..f5b93a86b9c7 100644
--- a/toolchain/check/testdata/impl/fail_extend_impl_type_as.carbon
+++ b/toolchain/check/testdata/impl/fail_extend_impl_type_as.carbon
@@ -54,6 +54,7 @@ class E {
 // CHECK:STDOUT: interface @I {
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
+// CHECK:STDOUT:   witness = ()
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl.1: i32 as I {
diff --git a/toolchain/check/testdata/impl/fail_extend_partially_defined_interface.carbon b/toolchain/check/testdata/impl/fail_extend_partially_defined_interface.carbon
index 64940b80c3fa..aeb451e5ccdd 100644
--- a/toolchain/check/testdata/impl/fail_extend_partially_defined_interface.carbon
+++ b/toolchain/check/testdata/impl/fail_extend_partially_defined_interface.carbon
@@ -34,6 +34,7 @@ interface I {
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .C = %C.decl
+// CHECK:STDOUT:   witness = ()
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl: C as I;
diff --git a/toolchain/check/testdata/impl/fail_impl_as_scope.carbon b/toolchain/check/testdata/impl/fail_impl_as_scope.carbon
index 4469ce182c49..ec62923d1e51 100644
--- a/toolchain/check/testdata/impl/fail_impl_as_scope.carbon
+++ b/toolchain/check/testdata/impl/fail_impl_as_scope.carbon
@@ -19,19 +19,23 @@ impl as Simple {
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %.1: type = interface_type @Simple [template]
+// CHECK:STDOUT:   %.2: type = assoc_entity_type @Simple, <function> [template]
+// CHECK:STDOUT:   %.3: <associated <function> in Simple> = assoc_entity element0, @Simple.%F [template]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: file {
 // CHECK:STDOUT:   package: <namespace> = namespace {.Simple = %Simple.decl} [template]
 // CHECK:STDOUT:   %Simple.decl = interface_decl @Simple, () [template = constants.%.1]
-// CHECK:STDOUT:   impl_decl @impl, (<unexpected instref inst+4>)
+// CHECK:STDOUT:   impl_decl @impl, (<unexpected instref inst+7>)
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: interface @Simple {
 // CHECK:STDOUT:   %F: <function> = fn_decl @F.1 [template]
+// CHECK:STDOUT:   %.loc8: <associated <function> in Simple> = assoc_entity element0, %F [template = constants.%.3]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
-// CHECK:STDOUT:   .F = %F
+// CHECK:STDOUT:   .F = %.loc8
+// CHECK:STDOUT:   witness = (%F)
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl: <error> as Simple {
diff --git a/toolchain/check/testdata/impl/fail_impl_bad_type.carbon b/toolchain/check/testdata/impl/fail_impl_bad_type.carbon
index 76b4792d951d..5d3456d53f73 100644
--- a/toolchain/check/testdata/impl/fail_impl_bad_type.carbon
+++ b/toolchain/check/testdata/impl/fail_impl_bad_type.carbon
@@ -27,6 +27,7 @@ impl true as I {}
 // CHECK:STDOUT: interface @I {
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
+// CHECK:STDOUT:   witness = ()
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl: <error> as I {
diff --git a/toolchain/check/testdata/impl/fail_redefinition.carbon b/toolchain/check/testdata/impl/fail_redefinition.carbon
index aba88d0a4ca9..b44c8ef20ff1 100644
--- a/toolchain/check/testdata/impl/fail_redefinition.carbon
+++ b/toolchain/check/testdata/impl/fail_redefinition.carbon
@@ -32,6 +32,7 @@ impl i32 as I {}
 // CHECK:STDOUT: interface @I {
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
+// CHECK:STDOUT:   witness = ()
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl: i32 as I {
diff --git a/toolchain/check/testdata/impl/extend_impl.carbon b/toolchain/check/testdata/impl/fail_todo_extend_impl.carbon
similarity index 56%
rename from toolchain/check/testdata/impl/extend_impl.carbon
rename to toolchain/check/testdata/impl/fail_todo_extend_impl.carbon
index 06bbd0321dd3..dfc22bc3fe13 100644
--- a/toolchain/check/testdata/impl/extend_impl.carbon
+++ b/toolchain/check/testdata/impl/fail_todo_extend_impl.carbon
@@ -15,20 +15,28 @@ class C {
 }
 
 fn G(c: C) {
-  // TODO: These don't do impl lookup, and so refer to the interface members
-  // rather than to the impl members.
+  // TODO: These should do impl lookup. Because that's not implemented yet, they
+  // refer to the interface members rather than to the impl members.
+  // CHECK:STDERR: fail_todo_extend_impl.carbon:[[@LINE+3]]:3: ERROR: Value of type `<associated <function> in HasF>` is not callable.
+  // CHECK:STDERR:   C.F();
+  // CHECK:STDERR:   ^~~~
   C.F();
+  // CHECK:STDERR: fail_todo_extend_impl.carbon:[[@LINE+3]]:3: ERROR: Value of type `<associated <function> in HasF>` is not callable.
+  // CHECK:STDERR:   c.F();
+  // CHECK:STDERR:   ^~~~
   c.F();
 }
 
-// CHECK:STDOUT: --- extend_impl.carbon
+// CHECK:STDOUT: --- fail_todo_extend_impl.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %.1: type = interface_type @HasF [template]
+// CHECK:STDOUT:   %.2: type = assoc_entity_type @HasF, <function> [template]
+// CHECK:STDOUT:   %.3: <associated <function> in HasF> = assoc_entity element0, @HasF.%F [template]
 // CHECK:STDOUT:   %C: type = class_type @C [template]
-// CHECK:STDOUT:   %.2: type = struct_type {} [template]
-// CHECK:STDOUT:   %.3: type = tuple_type () [template]
-// CHECK:STDOUT:   %.4: type = ptr_type {} [template]
+// CHECK:STDOUT:   %.4: type = struct_type {} [template]
+// CHECK:STDOUT:   %.5: type = tuple_type () [template]
+// CHECK:STDOUT:   %.6: type = ptr_type {} [template]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: file {
@@ -40,9 +48,11 @@ fn G(c: C) {
 // CHECK:STDOUT:
 // CHECK:STDOUT: interface @HasF {
 // CHECK:STDOUT:   %F: <function> = fn_decl @F.1 [template]
+// CHECK:STDOUT:   %.loc8: <associated <function> in HasF> = assoc_entity element0, %F [template = constants.%.3]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
-// CHECK:STDOUT:   .F = %F
+// CHECK:STDOUT:   .F = %.loc8
+// CHECK:STDOUT:   witness = (%F)
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl: C as HasF {
@@ -53,7 +63,7 @@ fn G(c: C) {
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @C {
-// CHECK:STDOUT:   impl_decl @impl, (<unexpected instref inst+6>)
+// CHECK:STDOUT:   impl_decl @impl, (<unexpected instref inst+9>)
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   extend name_scope1
@@ -69,11 +79,9 @@ fn G(c: C) {
 // CHECK:STDOUT: fn @G(%c: C) {
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   %C.ref: type = name_ref C, file.%C.decl [template = constants.%C]
-// CHECK:STDOUT:   %F.ref.loc20: <function> = name_ref F, @HasF.%F [template = @HasF.%F]
-// CHECK:STDOUT:   %.loc20: init () = call %F.ref.loc20()
+// CHECK:STDOUT:   %F.ref.loc23: <associated <function> in HasF> = name_ref F, @HasF.%.loc8 [template = constants.%.3]
 // CHECK:STDOUT:   %c.ref: C = name_ref c, %c
-// CHECK:STDOUT:   %F.ref.loc21: <function> = name_ref F, @HasF.%F [template = @HasF.%F]
-// CHECK:STDOUT:   %.loc21: init () = call %F.ref.loc21()
+// CHECK:STDOUT:   %F.ref.loc27: <associated <function> in HasF> = name_ref F, @HasF.%.loc8 [template = constants.%.3]
 // CHECK:STDOUT:   return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
diff --git a/toolchain/check/testdata/impl/impl_as.carbon b/toolchain/check/testdata/impl/impl_as.carbon
index c479537b3aa8..bedee6e304cf 100644
--- a/toolchain/check/testdata/impl/impl_as.carbon
+++ b/toolchain/check/testdata/impl/impl_as.carbon
@@ -18,8 +18,10 @@ class C {
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %.1: type = interface_type @Simple [template]
+// CHECK:STDOUT:   %.2: type = assoc_entity_type @Simple, <function> [template]
+// CHECK:STDOUT:   %.3: <associated <function> in Simple> = assoc_entity element0, @Simple.%F [template]
 // CHECK:STDOUT:   %C: type = class_type @C [template]
-// CHECK:STDOUT:   %.2: type = struct_type {} [template]
+// CHECK:STDOUT:   %.4: type = struct_type {} [template]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: file {
@@ -30,9 +32,11 @@ class C {
 // CHECK:STDOUT:
 // CHECK:STDOUT: interface @Simple {
 // CHECK:STDOUT:   %F: <function> = fn_decl @F.1 [template]
+// CHECK:STDOUT:   %.loc8: <associated <function> in Simple> = assoc_entity element0, %F [template = constants.%.3]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
-// CHECK:STDOUT:   .F = %F
+// CHECK:STDOUT:   .F = %.loc8
+// CHECK:STDOUT:   witness = (%F)
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl: C as Simple {
@@ -43,7 +47,7 @@ class C {
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @C {
-// CHECK:STDOUT:   impl_decl @impl, (<unexpected instref inst+6>)
+// CHECK:STDOUT:   impl_decl @impl, (<unexpected instref inst+9>)
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT: }
diff --git a/toolchain/check/testdata/impl/impl_forall.carbon b/toolchain/check/testdata/impl/impl_forall.carbon
index 15122ac05e96..6746bb33cdbf 100644
--- a/toolchain/check/testdata/impl/impl_forall.carbon
+++ b/toolchain/check/testdata/impl/impl_forall.carbon
@@ -16,19 +16,23 @@ impl forall [T:! type] T as Simple {
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %.1: type = interface_type @Simple [template]
+// CHECK:STDOUT:   %.2: type = assoc_entity_type @Simple, <function> [template]
+// CHECK:STDOUT:   %.3: <associated <function> in Simple> = assoc_entity element0, @Simple.%F [template]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: file {
 // CHECK:STDOUT:   package: <namespace> = namespace {.Simple = %Simple.decl} [template]
 // CHECK:STDOUT:   %Simple.decl = interface_decl @Simple, () [template = constants.%.1]
-// CHECK:STDOUT:   impl_decl @impl, (<unexpected instref inst+4>, <unexpected instref inst+5>, <unexpected instref inst+6>, <unexpected instref inst+7>)
+// CHECK:STDOUT:   impl_decl @impl, (<unexpected instref inst+7>, <unexpected instref inst+8>, <unexpected instref inst+9>, <unexpected instref inst+10>)
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: interface @Simple {
 // CHECK:STDOUT:   %F: <function> = fn_decl @F.1 [template]
+// CHECK:STDOUT:   %.loc8: <associated <function> in Simple> = assoc_entity element0, %F [template = constants.%.3]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
-// CHECK:STDOUT:   .F = %F
+// CHECK:STDOUT:   .F = %.loc8
+// CHECK:STDOUT:   witness = (%F)
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl: T as Simple {
diff --git a/toolchain/check/testdata/impl/redeclaration.carbon b/toolchain/check/testdata/impl/redeclaration.carbon
index 5b9016be7497..58e0949b7120 100644
--- a/toolchain/check/testdata/impl/redeclaration.carbon
+++ b/toolchain/check/testdata/impl/redeclaration.carbon
@@ -33,6 +33,7 @@ impl i32 as I {}
 // CHECK:STDOUT: interface @I {
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
+// CHECK:STDOUT:   witness = ()
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @impl: i32 as I {
diff --git a/toolchain/check/testdata/interface/as_type.carbon b/toolchain/check/testdata/interface/as_type.carbon
index 809821782a52..9c8c6e07f0e3 100644
--- a/toolchain/check/testdata/interface/as_type.carbon
+++ b/toolchain/check/testdata/interface/as_type.carbon
@@ -24,6 +24,7 @@ fn F(e: Empty) {}
 // CHECK:STDOUT: interface @Empty {
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
+// CHECK:STDOUT:   witness = ()
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @F(%e: Empty) {
diff --git a/toolchain/check/testdata/interface/basic.carbon b/toolchain/check/testdata/interface/basic.carbon
index 1cc4986d4cf9..f9397cc3116d 100644
--- a/toolchain/check/testdata/interface/basic.carbon
+++ b/toolchain/check/testdata/interface/basic.carbon
@@ -18,6 +18,8 @@ interface ForwardDeclared {
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %.1: type = interface_type @Empty [template]
 // CHECK:STDOUT:   %.2: type = interface_type @ForwardDeclared [template]
+// CHECK:STDOUT:   %.3: type = assoc_entity_type @ForwardDeclared, <function> [template]
+// CHECK:STDOUT:   %.4: <associated <function> in ForwardDeclared> = assoc_entity element0, @ForwardDeclared.%F [template]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: file {
@@ -30,13 +32,16 @@ interface ForwardDeclared {
 // CHECK:STDOUT: interface @Empty {
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
+// CHECK:STDOUT:   witness = ()
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: interface @ForwardDeclared {
 // CHECK:STDOUT:   %F: <function> = fn_decl @F [template]
+// CHECK:STDOUT:   %.loc13: <associated <function> in ForwardDeclared> = assoc_entity element0, %F [template = constants.%.4]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
-// CHECK:STDOUT:   .F = %F
+// CHECK:STDOUT:   .F = %.loc13
+// CHECK:STDOUT:   witness = (%F)
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @F();
diff --git a/toolchain/check/testdata/interface/fail_add_member_outside_definition.carbon b/toolchain/check/testdata/interface/fail_add_member_outside_definition.carbon
index 51afa962d2ec..6e25f79615b7 100644
--- a/toolchain/check/testdata/interface/fail_add_member_outside_definition.carbon
+++ b/toolchain/check/testdata/interface/fail_add_member_outside_definition.carbon
@@ -44,6 +44,7 @@ interface Outer {
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .F = file.%F
+// CHECK:STDOUT:   witness = ()
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: interface @Outer {
@@ -52,6 +53,7 @@ interface Outer {
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .Inner = %Inner.decl
+// CHECK:STDOUT:   witness = ()
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: interface @Inner {
@@ -59,6 +61,7 @@ interface Outer {
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .F = @Outer.%F
+// CHECK:STDOUT:   witness = ()
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @F.1() {
diff --git a/toolchain/check/testdata/interface/fail_as_type_of_type.carbon b/toolchain/check/testdata/interface/fail_as_type_of_type.carbon
index 0a90b75b77cf..71b1baa86acc 100644
--- a/toolchain/check/testdata/interface/fail_as_type_of_type.carbon
+++ b/toolchain/check/testdata/interface/fail_as_type_of_type.carbon
@@ -30,6 +30,7 @@ fn F(T:! Empty) {
 // CHECK:STDOUT: interface @Empty {
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
+// CHECK:STDOUT:   witness = ()
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @F(%T: Empty) {
diff --git a/toolchain/check/testdata/interface/fail_duplicate.carbon b/toolchain/check/testdata/interface/fail_duplicate.carbon
index 397c3fdfcda2..874ee6a98376 100644
--- a/toolchain/check/testdata/interface/fail_duplicate.carbon
+++ b/toolchain/check/testdata/interface/fail_duplicate.carbon
@@ -59,7 +59,8 @@ interface Class { }
 // CHECK:STDOUT:   %F: <function> = fn_decl @F [template]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
-// CHECK:STDOUT:   .F = %F
+// CHECK:STDOUT:   .F = <error>
+// CHECK:STDOUT:   witness = ()
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: interface @.1;
@@ -67,6 +68,7 @@ interface Class { }
 // CHECK:STDOUT: interface @.2 {
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
+// CHECK:STDOUT:   witness = ()
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @Class;
diff --git a/toolchain/check/testdata/interface/fail_lookup_undefined.carbon b/toolchain/check/testdata/interface/fail_lookup_undefined.carbon
index 137d4347e76f..725464e9b2a5 100644
--- a/toolchain/check/testdata/interface/fail_lookup_undefined.carbon
+++ b/toolchain/check/testdata/interface/fail_lookup_undefined.carbon
@@ -46,6 +46,8 @@ interface BeingDefined {
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %.1: type = interface_type @Undefined [template]
 // CHECK:STDOUT:   %.2: type = interface_type @BeingDefined [template]
+// CHECK:STDOUT:   %.3: type = assoc_entity_type @BeingDefined, <function> [template]
+// CHECK:STDOUT:   %.4: <associated <function> in BeingDefined> = assoc_entity element0, @BeingDefined.%H [template]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: file {
@@ -60,10 +62,12 @@ interface BeingDefined {
 // CHECK:STDOUT:
 // CHECK:STDOUT: interface @BeingDefined {
 // CHECK:STDOUT:   %H: <function> = fn_decl @H [template]
+// CHECK:STDOUT:   %.loc37: <associated <function> in BeingDefined> = assoc_entity element0, %H [template = constants.%.4]
 // CHECK:STDOUT:   %.loc41: <function> = fn_decl @.2 [template]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
-// CHECK:STDOUT:   .H = %H
+// CHECK:STDOUT:   .H = %.loc37
+// CHECK:STDOUT:   witness = (%H)
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @.1();
diff --git a/toolchain/check/testdata/interface/member_lookup.carbon b/toolchain/check/testdata/interface/fail_member_lookup.carbon
similarity index 61%
rename from toolchain/check/testdata/interface/member_lookup.carbon
rename to toolchain/check/testdata/interface/fail_member_lookup.carbon
index b3e960b05586..42db38287a87 100644
--- a/toolchain/check/testdata/interface/member_lookup.carbon
+++ b/toolchain/check/testdata/interface/fail_member_lookup.carbon
@@ -7,15 +7,18 @@
 interface Interface { fn F(); }
 
 fn F() {
-  // TODO: This should not be valid by itself.
+  // CHECK:STDERR: fail_member_lookup.carbon:[[@LINE+3]]:3: ERROR: Value of type `<associated <function> in Interface>` is not callable.
+  // CHECK:STDERR:   Interface.F();
+  // CHECK:STDERR:   ^~~~~~~~~~~~
   Interface.F();
 }
 
-// CHECK:STDOUT: --- member_lookup.carbon
+// CHECK:STDOUT: --- fail_member_lookup.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %.1: type = interface_type @Interface [template]
-// CHECK:STDOUT:   %.2: type = tuple_type () [template]
+// CHECK:STDOUT:   %.2: type = assoc_entity_type @Interface, <function> [template]
+// CHECK:STDOUT:   %.3: <associated <function> in Interface> = assoc_entity element0, @Interface.%F [template]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: file {
@@ -26,9 +29,11 @@ fn F() {
 // CHECK:STDOUT:
 // CHECK:STDOUT: interface @Interface {
 // CHECK:STDOUT:   %F: <function> = fn_decl @F.1 [template]
+// CHECK:STDOUT:   %.loc7: <associated <function> in Interface> = assoc_entity element0, %F [template = constants.%.3]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
-// CHECK:STDOUT:   .F = %F
+// CHECK:STDOUT:   .F = %.loc7
+// CHECK:STDOUT:   witness = (%F)
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @F.1();
@@ -36,8 +41,7 @@ fn F() {
 // CHECK:STDOUT: fn @F.2() {
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   %Interface.ref: type = name_ref Interface, file.%Interface.decl [template = constants.%.1]
-// CHECK:STDOUT:   %F.ref: <function> = name_ref F, @Interface.%F [template = @Interface.%F]
-// CHECK:STDOUT:   %.loc11: init () = call %F.ref()
+// CHECK:STDOUT:   %F.ref: <associated <function> in Interface> = name_ref F, @Interface.%.loc7 [template = constants.%.3]
 // CHECK:STDOUT:   return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
diff --git a/toolchain/check/testdata/interface/fail_modifiers.carbon b/toolchain/check/testdata/interface/fail_modifiers.carbon
index 4d4c96fbc6b1..5828c73284c8 100644
--- a/toolchain/check/testdata/interface/fail_modifiers.carbon
+++ b/toolchain/check/testdata/interface/fail_modifiers.carbon
@@ -46,6 +46,7 @@ protected interface Protected;
 // CHECK:STDOUT: interface @Abstract {
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
+// CHECK:STDOUT:   witness = ()
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: interface @Default;
@@ -53,6 +54,7 @@ protected interface Protected;
 // CHECK:STDOUT: interface @Virtual {
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
+// CHECK:STDOUT:   witness = ()
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: interface @Protected;
diff --git a/toolchain/check/testdata/interface/fail_redeclare_member.carbon b/toolchain/check/testdata/interface/fail_redeclare_member.carbon
new file mode 100644
index 000000000000..3aa22a0dd6ce
--- /dev/null
+++ b/toolchain/check/testdata/interface/fail_redeclare_member.carbon
@@ -0,0 +1,44 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+
+interface Interface {
+  fn F();
+  // CHECK:STDERR: fail_redeclare_member.carbon:[[@LINE+6]]:3: ERROR: Duplicate name being declared in the same scope.
+  // CHECK:STDERR:   fn F();
+  // CHECK:STDERR:   ^~~~~~~
+  // CHECK:STDERR: fail_redeclare_member.carbon:[[@LINE-4]]:3: Name is previously declared here.
+  // CHECK:STDERR:   fn F();
+  // CHECK:STDERR:   ^~~~~~~
+  fn F();
+}
+
+// CHECK:STDOUT: --- fail_redeclare_member.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %.1: type = interface_type @Interface [template]
+// CHECK:STDOUT:   %.2: type = assoc_entity_type @Interface, <function> [template]
+// CHECK:STDOUT:   %.3: <associated <function> in Interface> = assoc_entity element0, @Interface.%F [template]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace {.Interface = %Interface.decl} [template]
+// CHECK:STDOUT:   %Interface.decl = interface_decl @Interface, () [template = constants.%.1]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: interface @Interface {
+// CHECK:STDOUT:   %F: <function> = fn_decl @F [template]
+// CHECK:STDOUT:   %.loc8: <associated <function> in Interface> = assoc_entity element0, %F [template = constants.%.3]
+// CHECK:STDOUT:   %.loc15: <function> = fn_decl @.1 [template]
+// CHECK:STDOUT:
+// CHECK:STDOUT: !members:
+// CHECK:STDOUT:   .F = %.loc8
+// CHECK:STDOUT:   witness = (%F)
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @F();
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @.1();
+// CHECK:STDOUT:
diff --git a/toolchain/check/testdata/interface/fail_todo_define_out_of_line.carbon b/toolchain/check/testdata/interface/fail_todo_define_out_of_line.carbon
new file mode 100644
index 000000000000..05b4fbc39449
--- /dev/null
+++ b/toolchain/check/testdata/interface/fail_todo_define_out_of_line.carbon
@@ -0,0 +1,51 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+
+interface Interface {
+  // CHECK:STDERR: fail_todo_define_out_of_line.carbon:[[@LINE+3]]:3: ERROR: Semantics TODO: `interface modifier`.
+  // CHECK:STDERR:   default fn F();
+  // CHECK:STDERR:   ^~~~~~~
+  default fn F();
+}
+
+// CHECK:STDERR: fail_todo_define_out_of_line.carbon:[[@LINE+6]]:1: ERROR: Duplicate name being declared in the same scope.
+// CHECK:STDERR: fn Interface.F() {}
+// CHECK:STDERR: ^~~~~~~~~~~~~~~~~~
+// CHECK:STDERR: fail_todo_define_out_of_line.carbon:[[@LINE-6]]:3: Name is previously declared here.
+// CHECK:STDERR:   default fn F();
+// CHECK:STDERR:   ^~~~~~~~~~~~~~~
+fn Interface.F() {}
+
+// CHECK:STDOUT: --- fail_todo_define_out_of_line.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %.1: type = interface_type @Interface [template]
+// CHECK:STDOUT:   %.2: type = assoc_entity_type @Interface, <function> [template]
+// CHECK:STDOUT:   %.3: <associated <function> in Interface> = assoc_entity element0, @Interface.%F [template]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace {.Interface = %Interface.decl} [template]
+// CHECK:STDOUT:   %Interface.decl = interface_decl @Interface, () [template = constants.%.1]
+// CHECK:STDOUT:   %.loc20: <function> = fn_decl @.1 [template]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: interface @Interface {
+// CHECK:STDOUT:   %F: <function> = fn_decl @F [template]
+// CHECK:STDOUT:   %.loc11: <associated <function> in Interface> = assoc_entity element0, %F [template = constants.%.3]
+// CHECK:STDOUT:
+// CHECK:STDOUT: !members:
+// CHECK:STDOUT:   .F = %.loc11
+// CHECK:STDOUT:   witness = (%F)
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @F();
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @.1() {
+// CHECK:STDOUT: !entry:
+// CHECK:STDOUT:   return
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
diff --git a/toolchain/check/testdata/interface/fail_todo_facet_lookup.carbon b/toolchain/check/testdata/interface/fail_todo_facet_lookup.carbon
index 25dd4521f403..a06cede5b98a 100644
--- a/toolchain/check/testdata/interface/fail_todo_facet_lookup.carbon
+++ b/toolchain/check/testdata/interface/fail_todo_facet_lookup.carbon
@@ -17,7 +17,9 @@ fn CallStatic(T:! Interface) {
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %.1: type = interface_type @Interface [template]
-// CHECK:STDOUT:   %.2: type = tuple_type () [template]
+// CHECK:STDOUT:   %.2: type = assoc_entity_type @Interface, <function> [template]
+// CHECK:STDOUT:   %.3: <associated <function> in Interface> = assoc_entity element0, @Interface.%F [template]
+// CHECK:STDOUT:   %.4: type = tuple_type () [template]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: file {
@@ -28,9 +30,11 @@ fn CallStatic(T:! Interface) {
 // CHECK:STDOUT:
 // CHECK:STDOUT: interface @Interface {
 // CHECK:STDOUT:   %F: <function> = fn_decl @F [template]
+// CHECK:STDOUT:   %.loc7: <associated <function> in Interface> = assoc_entity element0, %F [template = constants.%.3]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
-// CHECK:STDOUT:   .F = %F
+// CHECK:STDOUT:   .F = %.loc7
+// CHECK:STDOUT:   witness = (%F)
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @F();
diff --git a/toolchain/check/testdata/interface/fail_todo_modifiers.carbon b/toolchain/check/testdata/interface/fail_todo_modifiers.carbon
index b297c3a8b7b3..d518a1ce6a10 100644
--- a/toolchain/check/testdata/interface/fail_todo_modifiers.carbon
+++ b/toolchain/check/testdata/interface/fail_todo_modifiers.carbon
@@ -25,27 +25,34 @@ private interface Private {
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %.1: type = interface_type @Modifiers [template]
-// CHECK:STDOUT:   %.2: type = interface_type @Private [template]
+// CHECK:STDOUT:   %.2: type = assoc_entity_type @Modifiers, <function> [template]
+// CHECK:STDOUT:   %.3: <associated <function> in Modifiers> = assoc_entity element0, @Modifiers.%Final [template]
+// CHECK:STDOUT:   %.4: <associated <function> in Modifiers> = assoc_entity element1, @Modifiers.%Default [template]
+// CHECK:STDOUT:   %.5: type = interface_type @Private [template]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: file {
 // CHECK:STDOUT:   package: <namespace> = namespace {.Modifiers = %Modifiers.decl, .Private = %Private.decl} [template]
 // CHECK:STDOUT:   %Modifiers.decl = interface_decl @Modifiers, () [template = constants.%.1]
-// CHECK:STDOUT:   %Private.decl = interface_decl @Private, () [template = constants.%.2]
+// CHECK:STDOUT:   %Private.decl = interface_decl @Private, () [template = constants.%.5]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: interface @Modifiers {
 // CHECK:STDOUT:   %Final: <function> = fn_decl @Final [template]
+// CHECK:STDOUT:   %.loc11: <associated <function> in Modifiers> = assoc_entity element0, %Final [template = constants.%.3]
 // CHECK:STDOUT:   %Default: <function> = fn_decl @Default [template]
+// CHECK:STDOUT:   %.loc15: <associated <function> in Modifiers> = assoc_entity element1, %Default [template = constants.%.4]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
-// CHECK:STDOUT:   .Final = %Final
-// CHECK:STDOUT:   .Default = %Default
+// CHECK:STDOUT:   .Final = %.loc11
+// CHECK:STDOUT:   .Default = %.loc15
+// CHECK:STDOUT:   witness = (%Final, %Default)
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: interface @Private {
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
+// CHECK:STDOUT:   witness = ()
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @Final() {
diff --git a/toolchain/check/testdata/interface/import.carbon b/toolchain/check/testdata/interface/import.carbon
index f6e0a6b4f0f5..db881cf961fe 100644
--- a/toolchain/check/testdata/interface/import.carbon
+++ b/toolchain/check/testdata/interface/import.carbon
@@ -35,9 +35,11 @@ var f: ForwardDeclared* = &f_ref.f;
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %.1: type = interface_type @Empty [template]
 // CHECK:STDOUT:   %.2: type = interface_type @ForwardDeclared [template]
-// CHECK:STDOUT:   %.3: type = struct_type {.f: ForwardDeclared} [template]
-// CHECK:STDOUT:   %.4: type = tuple_type () [template]
-// CHECK:STDOUT:   %.5: type = struct_type {.f: ()} [template]
+// CHECK:STDOUT:   %.3: type = assoc_entity_type @ForwardDeclared, <function> [template]
+// CHECK:STDOUT:   %.4: <associated <function> in ForwardDeclared> = assoc_entity element0, @ForwardDeclared.%F [template]
+// CHECK:STDOUT:   %.5: type = struct_type {.f: ForwardDeclared} [template]
+// CHECK:STDOUT:   %.6: type = tuple_type () [template]
+// CHECK:STDOUT:   %.7: type = struct_type {.f: ()} [template]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: file {
@@ -46,7 +48,7 @@ var f: ForwardDeclared* = &f_ref.f;
 // CHECK:STDOUT:   %ForwardDeclared.decl.loc7 = interface_decl @ForwardDeclared, () [template = constants.%.2]
 // CHECK:STDOUT:   %ForwardDeclared.decl.loc9 = interface_decl @ForwardDeclared, () [template = constants.%.2]
 // CHECK:STDOUT:   %ForwardDeclared.ref: type = name_ref ForwardDeclared, %ForwardDeclared.decl.loc7 [template = constants.%.2]
-// CHECK:STDOUT:   %.loc13: type = struct_type {.f: ForwardDeclared} [template = constants.%.3]
+// CHECK:STDOUT:   %.loc13: type = struct_type {.f: ForwardDeclared} [template = constants.%.5]
 // CHECK:STDOUT:   %f_ref.var: ref {.f: ForwardDeclared} = var f_ref
 // CHECK:STDOUT:   %f_ref: ref {.f: ForwardDeclared} = bind_name f_ref, %f_ref.var
 // CHECK:STDOUT: }
@@ -54,13 +56,16 @@ var f: ForwardDeclared* = &f_ref.f;
 // CHECK:STDOUT: interface @Empty {
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
+// CHECK:STDOUT:   witness = ()
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: interface @ForwardDeclared {
 // CHECK:STDOUT:   %F: <function> = fn_decl @F [template]
+// CHECK:STDOUT:   %.loc10: <associated <function> in ForwardDeclared> = assoc_entity element0, %F [template = constants.%.4]
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
-// CHECK:STDOUT:   .F = %F
+// CHECK:STDOUT:   .F = %.loc10
+// CHECK:STDOUT:   witness = (%F)
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @F();
@@ -80,7 +85,7 @@ var f: ForwardDeclared* = &f_ref.f;
 // CHECK:STDOUT:   package: <namespace> = namespace {.Empty = %import_ref.1, .ForwardDeclared = %import_ref.2, .f_ref = %import_ref.3, .UseEmpty = %UseEmpty, .UseForwardDeclared = %UseForwardDeclared, .f = %f} [template]
 // CHECK:STDOUT:   %import_ref.1: type = import_ref ir1, inst+1, used [template = constants.%.1]
 // CHECK:STDOUT:   %import_ref.2: type = import_ref ir1, inst+3, used [template = constants.%.3]
-// CHECK:STDOUT:   %import_ref.3: ref {.f: ForwardDeclared} = import_ref ir1, inst+16, used
+// CHECK:STDOUT:   %import_ref.3: ref {.f: ForwardDeclared} = import_ref ir1, inst+19, used
 // CHECK:STDOUT:   %UseEmpty: <function> = fn_decl @UseEmpty [template]
 // CHECK:STDOUT:   %UseForwardDeclared: <function> = fn_decl @UseForwardDeclared [template]
 // CHECK:STDOUT:   %ForwardDeclared.ref: type = name_ref ForwardDeclared, %import_ref.2 [template = constants.%.3]
@@ -92,13 +97,16 @@ var f: ForwardDeclared* = &f_ref.f;
 // CHECK:STDOUT: interface @Empty {
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
+// CHECK:STDOUT:   witness = ()
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: interface @ForwardDeclared {
-// CHECK:STDOUT:   %import_ref = import_ref ir1, inst+6, unused
+// CHECK:STDOUT:   %import_ref.1 = import_ref ir1, inst+8, unused
+// CHECK:STDOUT:   %import_ref.2 = import_ref ir1, inst+6, unused
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
-// CHECK:STDOUT:   .F = %import_ref
+// CHECK:STDOUT:   .F = %import_ref.1
+// CHECK:STDOUT:   witness = (%import_ref.2)
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @UseEmpty(%e: Empty) {
diff --git a/toolchain/lower/file_context.cpp b/toolchain/lower/file_context.cpp
index eeaade664eb6..35b2fc648289 100644
--- a/toolchain/lower/file_context.cpp
+++ b/toolchain/lower/file_context.cpp
@@ -11,6 +11,7 @@
 #include "toolchain/sem_ir/entry_point.h"
 #include "toolchain/sem_ir/file.h"
 #include "toolchain/sem_ir/inst.h"
+#include "toolchain/sem_ir/typed_insts.h"
 
 namespace Carbon::Lower {
 
@@ -66,6 +67,10 @@ auto FileContext::GetGlobal(SemIR::InstId inst_id) -> llvm::Value* {
     return GetFunction(function_decl->function_id);
   }
 
+  if (target.Is<SemIR::AssociatedEntity>()) {
+    return llvm::ConstantStruct::getAnon(llvm_context(), {});
+  }
+
   if (target.type_id() == SemIR::TypeId::TypeType) {
     return GetTypeAsValue();
   }
@@ -274,6 +279,10 @@ auto FileContext::BuildType(SemIR::InstId inst_id) -> llvm::Type* {
           GetType(array_type.element_type_id),
           sem_ir_->GetArrayBoundValue(array_type.bound_id));
     }
+    case SemIR::AssociatedEntityType::Kind:
+      // No runtime operations are provided on an associated entity name, so use
+      // an empty representation.
+      return llvm::StructType::get(*llvm_context_);
     case SemIR::BindSymbolicName::Kind:
       // Treat non-monomorphized type bindings as opaque.
       return llvm::StructType::get(*llvm_context_);
diff --git a/toolchain/lower/handle.cpp b/toolchain/lower/handle.cpp
index 5ab88a712bbd..290bfbafb96b 100644
--- a/toolchain/lower/handle.cpp
+++ b/toolchain/lower/handle.cpp
@@ -60,6 +60,12 @@ auto HandleAssign(FunctionContext& context, SemIR::InstId /*inst_id*/,
   context.FinishInit(storage_type_id, inst.lhs_id, inst.rhs_id);
 }
 
+auto HandleAssociatedEntity(FunctionContext& /*context*/,
+                            SemIR::InstId /*inst_id*/,
+                            SemIR::AssociatedEntity inst) -> void {
+  FatalErrorIfEncountered(inst);
+}
+
 auto HandleBindAlias(FunctionContext& context, SemIR::InstId inst_id,
                      SemIR::BindAlias inst) -> void {
   auto type_inst_id = context.sem_ir().types().GetInstId(inst.type_id);
@@ -229,9 +235,9 @@ auto HandleInitializeFrom(FunctionContext& context, SemIR::InstId /*inst_id*/,
 }
 
 auto HandleInterfaceDecl(FunctionContext& /*context*/,
-                         SemIR::InstId /*inst_id*/,
-                         SemIR::InterfaceDecl /*inst*/) -> void {
-  // No action to perform.
+                         SemIR::InstId /*inst_id*/, SemIR::InterfaceDecl inst)
+    -> void {
+  FatalErrorIfEncountered(inst);
 }
 
 auto HandleIntLiteral(FunctionContext& context, SemIR::InstId inst_id,
diff --git a/toolchain/lower/handle_type.cpp b/toolchain/lower/handle_type.cpp
index 96279ad2471d..1174736822bc 100644
--- a/toolchain/lower/handle_type.cpp
+++ b/toolchain/lower/handle_type.cpp
@@ -3,6 +3,7 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
 #include "toolchain/lower/function_context.h"
+#include "toolchain/sem_ir/typed_insts.h"
 
 namespace Carbon::Lower {
 
@@ -11,6 +12,11 @@ auto HandleArrayType(FunctionContext& context, SemIR::InstId inst_id,
   context.SetLocal(inst_id, context.GetTypeAsValue());
 }
 
+auto HandleAssociatedEntityType(FunctionContext& context, SemIR::InstId inst_id,
+                                SemIR::AssociatedEntityType /*inst*/) -> void {
+  context.SetLocal(inst_id, context.GetTypeAsValue());
+}
+
 auto HandleClassType(FunctionContext& context, SemIR::InstId inst_id,
                      SemIR::ClassType /*inst*/) -> void {
   context.SetLocal(inst_id, context.GetTypeAsValue());
diff --git a/toolchain/lower/testdata/interface/assoc.carbon b/toolchain/lower/testdata/interface/assoc.carbon
new file mode 100644
index 000000000000..ae427a84623e
--- /dev/null
+++ b/toolchain/lower/testdata/interface/assoc.carbon
@@ -0,0 +1,20 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+
+interface I {
+  fn Assoc();
+}
+
+fn F() { I.Assoc; }
+
+// CHECK:STDOUT: ; ModuleID = 'assoc.carbon'
+// CHECK:STDOUT: source_filename = "assoc.carbon"
+// CHECK:STDOUT:
+// CHECK:STDOUT: declare void @Assoc()
+// CHECK:STDOUT:
+// CHECK:STDOUT: define void @F() {
+// CHECK:STDOUT:   ret void
+// CHECK:STDOUT: }
diff --git a/toolchain/lower/testdata/interface/basic.carbon b/toolchain/lower/testdata/interface/basic.carbon
new file mode 100644
index 000000000000..9fb909e13883
--- /dev/null
+++ b/toolchain/lower/testdata/interface/basic.carbon
@@ -0,0 +1,31 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+
+interface I {
+  fn Assoc();
+}
+
+// There are no interesting runtime operations here, but there's no rule saying
+// you can't pass a facet around at runtime, so make sure it works.
+fn F(T: I) -> I { return T; }
+
+interface J;
+
+// Declared-but-not-defined interfaces are still complete types.
+fn G(T: J) {}
+
+// CHECK:STDOUT: ; ModuleID = 'basic.carbon'
+// CHECK:STDOUT: source_filename = "basic.carbon"
+// CHECK:STDOUT:
+// CHECK:STDOUT: declare void @Assoc()
+// CHECK:STDOUT:
+// CHECK:STDOUT: define void @F() {
+// CHECK:STDOUT:   ret void
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: define void @G() {
+// CHECK:STDOUT:   ret void
+// CHECK:STDOUT: }
diff --git a/toolchain/sem_ir/file.cpp b/toolchain/sem_ir/file.cpp
index d7b5fa7c11cc..107c67615298 100644
--- a/toolchain/sem_ir/file.cpp
+++ b/toolchain/sem_ir/file.cpp
@@ -191,6 +191,7 @@ auto File::OutputYaml(bool include_builtins) const -> Yaml::OutputMapping {
 static auto GetTypePrecedence(InstKind kind) -> int {
   switch (kind) {
     case ArrayType::Kind:
+    case AssociatedEntityType::Kind:
     case BindAlias::Kind:
     case BindSymbolicName::Kind:
     case Builtin::Kind:
@@ -212,6 +213,7 @@ static auto GetTypePrecedence(InstKind kind) -> int {
     case ArrayIndex::Kind:
     case ArrayInit::Kind:
     case Assign::Kind:
+    case AssociatedEntity::Kind:
     case BaseDecl::Kind:
     case BindName::Kind:
     case BindValue::Kind:
@@ -316,6 +318,20 @@ static auto StringifyTypeExprImpl(const SemIR::File& outer_sem_ir,
         }
         break;
       }
+      case AssociatedEntityType::Kind: {
+        auto assoc = inst.As<AssociatedEntityType>();
+        if (step.index == 0) {
+          out << "<associated ";
+          steps.push_back(step.Next());
+          push_inst_id(sem_ir.types().GetInstId(assoc.entity_type_id));
+        } else {
+          auto interface_name_id =
+              sem_ir.interfaces().Get(assoc.interface_id).name_id;
+          out << " in " << sem_ir.names().GetFormatted(interface_name_id)
+              << ">";
+        }
+        break;
+      }
       case BindAlias::Kind:
       case BindSymbolicName::Kind: {
         auto name_id = inst.As<AnyBindName>().bind_name_id;
@@ -437,6 +453,7 @@ static auto StringifyTypeExprImpl(const SemIR::File& outer_sem_ir,
       case ArrayIndex::Kind:
       case ArrayInit::Kind:
       case Assign::Kind:
+      case AssociatedEntity::Kind:
       case BaseDecl::Kind:
       case BindName::Kind:
       case BindValue::Kind:
@@ -552,6 +569,8 @@ auto GetExprCategory(const File& file, InstId inst_id) -> ExprCategory {
       case AddrOf::Kind:
       case AddrPattern::Kind:
       case ArrayType::Kind:
+      case AssociatedEntity::Kind:
+      case AssociatedEntityType::Kind:
       case BindSymbolicName::Kind:
       case BindValue::Kind:
       case BlockArg::Kind:
diff --git a/toolchain/sem_ir/formatter.cpp b/toolchain/sem_ir/formatter.cpp
index b5d38ae9f650..825da1f71dd9 100644
--- a/toolchain/sem_ir/formatter.cpp
+++ b/toolchain/sem_ir/formatter.cpp
@@ -649,6 +649,8 @@ class Formatter {
       FormatCodeBlock(interface_info.body_block_id);
       out_ << "\n!members:";
       FormatNameScope(interface_info.scope_id, "", "\n  ");
+      out_ << "\n  witness = ";
+      FormatArg(interface_info.associated_entities_id);
       out_ << "\n}\n";
     } else {
       out_ << ";\n";
diff --git a/toolchain/sem_ir/inst.h b/toolchain/sem_ir/inst.h
index 0114325ba317..ae010c1ee7b8 100644
--- a/toolchain/sem_ir/inst.h
+++ b/toolchain/sem_ir/inst.h
@@ -343,6 +343,16 @@ class InstStore {
     return Get(inst_id).TryAs<InstT>();
   }
 
+  // Returns the requested instruction as the specified type, if it is valid and
+  // of that type. Otherwise returns nullopt.
+  template <typename InstT>
+  auto TryGetAsIfValid(InstId inst_id) const -> std::optional<InstT> {
+    if (!inst_id.is_valid()) {
+      return std::nullopt;
+    }
+    return TryGetAs<InstT>(inst_id);
+  }
+
   auto GetParseNode(InstId inst_id) const -> Parse::NodeId {
     return parse_nodes_[inst_id.index];
   }
diff --git a/toolchain/sem_ir/inst_kind.def b/toolchain/sem_ir/inst_kind.def
index 92abe2abb93d..be427faa6824 100644
--- a/toolchain/sem_ir/inst_kind.def
+++ b/toolchain/sem_ir/inst_kind.def
@@ -23,6 +23,8 @@ CARBON_SEM_IR_INST_KIND(ArrayIndex)
 CARBON_SEM_IR_INST_KIND(ArrayInit)
 CARBON_SEM_IR_INST_KIND(ArrayType)
 CARBON_SEM_IR_INST_KIND(Assign)
+CARBON_SEM_IR_INST_KIND(AssociatedEntity)
+CARBON_SEM_IR_INST_KIND(AssociatedEntityType)
 CARBON_SEM_IR_INST_KIND(BaseDecl)
 CARBON_SEM_IR_INST_KIND(BindAlias)
 CARBON_SEM_IR_INST_KIND(BindName)
diff --git a/toolchain/sem_ir/interface.h b/toolchain/sem_ir/interface.h
index de625009a2d9..9aecae23c2ff 100644
--- a/toolchain/sem_ir/interface.h
+++ b/toolchain/sem_ir/interface.h
@@ -18,7 +18,13 @@ struct Interface : public Printable<Interface> {
 
   // Determines whether this interface has been fully defined. This is false
   // until we reach the `}` of the interface definition.
-  auto is_defined() const -> bool { return defined; }
+  auto is_defined() const -> bool { return associated_entities_id.is_valid(); }
+
+  // Determines whether we're currently defining the interface. This is true
+  // between the braces of the interface.
+  auto is_being_defined() const -> bool {
+    return definition_id.is_valid() && !is_defined();
+  }
 
   // The following members always have values, and do not change throughout the
   // lifetime of the interface.
@@ -41,7 +47,7 @@ struct Interface : public Printable<Interface> {
   InstBlockId body_block_id = InstBlockId::Invalid;
 
   // The following members are set at the `}` of the interface definition.
-  bool defined = false;
+  InstBlockId associated_entities_id = InstBlockId::Invalid;
 };
 
 }  // namespace Carbon::SemIR
diff --git a/toolchain/sem_ir/name_scope.h b/toolchain/sem_ir/name_scope.h
index 3f73348d6681..cfc16aaa9859 100644
--- a/toolchain/sem_ir/name_scope.h
+++ b/toolchain/sem_ir/name_scope.h
@@ -95,6 +95,16 @@ class NameScopeStore {
     return values_.Get(scope_id);
   }
 
+  // Returns the instruction owning the requested name scope, or an invalid
+  // instruction if the scope is either invalid or has no associated
+  // instruction.
+  auto GetInstIdIfValid(NameScopeId scope_id) const -> InstId {
+    if (!scope_id.is_valid()) {
+      return InstId::Invalid;
+    }
+    return Get(scope_id).inst_id;
+  }
+
   auto OutputYaml() const -> Yaml::OutputMapping {
     return values_.OutputYaml();
   }
diff --git a/toolchain/sem_ir/typed_insts.h b/toolchain/sem_ir/typed_insts.h
index 93a46fd375b4..f6d189587485 100644
--- a/toolchain/sem_ir/typed_insts.h
+++ b/toolchain/sem_ir/typed_insts.h
@@ -154,6 +154,32 @@ struct Assign {
   InstId rhs_id;
 };
 
+// An associated entity declared in an interface. This is either an associated
+// function or a non-function associated constant such as an associated type.
+// This represents the entity before impl lookup is performed, and identifies
+// the slot within a witness where the constant value will be found.
+struct AssociatedEntity {
+  static constexpr auto Kind =
+      InstKind::AssociatedEntity.Define<Parse::NodeId>("assoc_entity");
+
+  // The type of the associated entity. This is an AssociatedEntityType.
+  TypeId type_id;
+  ElementIndex index;
+  InstId decl_id;
+};
+
+// The type of an expression that names an associated entity, such as
+// `InterfaceName.Function`.
+struct AssociatedEntityType {
+  static constexpr auto Kind =
+      InstKind::AssociatedEntityType.Define<Parse::InvalidNodeId>(
+          "assoc_entity_type");
+
+  TypeId type_id;
+  InterfaceId interface_id;
+  TypeId entity_type_id;
+};
+
 // A base in a class, of the form `base: base_type;`. A base class is an
 // element of the derived class, and the type of the `BaseDecl` instruction is
 // an `UnboundElementType`.