diff --git a/toolchain/lowering/lowering_context.cpp b/toolchain/lowering/lowering_context.cpp
index ef13e3d19874..8dfbeb720173 100644
--- a/toolchain/lowering/lowering_context.cpp
+++ b/toolchain/lowering/lowering_context.cpp
@@ -5,6 +5,8 @@
 #include "toolchain/lowering/lowering_context.h"
 
 #include "common/vlog.h"
+#include "llvm/ADT/STLExtras.h"
+#include "llvm/ADT/Sequence.h"
 #include "toolchain/lowering/lowering_function_context.h"
 #include "toolchain/semantics/semantics_ir.h"
 #include "toolchain/semantics/semantics_node.h"
@@ -31,20 +33,20 @@ auto LoweringContext::Run() -> std::unique_ptr<llvm::Module> {
   // Lower types.
   auto types = semantics_ir_->types();
   types_.resize_for_overwrite(types.size());
-  for (int i = 0; i < static_cast<int>(types.size()); ++i) {
-    types_[i] = BuildType(types[i]);
+  for (auto [i, type] : llvm::enumerate(types)) {
+    types_[i] = BuildType(type);
   }
 
   // Lower function declarations.
   functions_.resize_for_overwrite(semantics_ir_->functions_size());
-  for (int i = 0; i < semantics_ir_->functions_size(); ++i) {
+  for (auto i : llvm::seq(semantics_ir_->functions_size())) {
     functions_[i] = BuildFunctionDeclaration(SemanticsFunctionId(i));
   }
 
   // TODO: Lower global variable declarations.
 
   // Lower function definitions.
-  for (int i = 0; i < semantics_ir_->functions_size(); ++i) {
+  for (auto i : llvm::seq(semantics_ir_->functions_size())) {
     BuildFunctionDefinition(SemanticsFunctionId(i));
   }
 
@@ -61,8 +63,8 @@ auto LoweringContext::BuildFunctionDeclaration(SemanticsFunctionId function_id)
   auto param_refs = semantics_ir().GetNodeBlock(function.param_refs_id);
   llvm::SmallVector<llvm::Type*> args;
   args.resize_for_overwrite(param_refs.size());
-  for (int i = 0; i < static_cast<int>(param_refs.size()); ++i) {
-    args[i] = GetType(semantics_ir().GetNode(param_refs[i]).type_id());
+  for (auto [i, param_ref] : llvm::enumerate(param_refs)) {
+    args[i] = GetType(semantics_ir().GetNode(param_ref).type_id());
   }
 
   // If return type is not valid, the function does not have a return type.
@@ -77,8 +79,8 @@ auto LoweringContext::BuildFunctionDeclaration(SemanticsFunctionId function_id)
       semantics_ir().GetString(function.name_id), llvm_module());
 
   // Set parameter names.
-  for (int i = 0; i < static_cast<int>(param_refs.size()); ++i) {
-    auto name_id = semantics_ir().GetNode(param_refs[i]).GetAsVarStorage();
+  for (auto [i, param_ref] : llvm::enumerate(param_refs)) {
+    auto name_id = semantics_ir().GetNode(param_ref).GetAsVarStorage();
     llvm_function->getArg(i)->setName(semantics_ir().GetString(name_id));
   }
 
@@ -99,8 +101,8 @@ auto LoweringContext::BuildFunctionDefinition(SemanticsFunctionId function_id)
 
   // Add parameters to locals.
   auto param_refs = semantics_ir().GetNodeBlock(function.param_refs_id);
-  for (int i = 0; i < static_cast<int>(param_refs.size()); ++i) {
-    function_lowering.SetLocal(param_refs[i], llvm_function->getArg(i));
+  for (auto [i, param_ref] : llvm::enumerate(param_refs)) {
+    function_lowering.SetLocal(param_ref, llvm_function->getArg(i));
   }
 
   // Lower all blocks.
diff --git a/toolchain/lowering/lowering_handle.cpp b/toolchain/lowering/lowering_handle.cpp
index 691dbe5f3173..a2fadd74d4c9 100644
--- a/toolchain/lowering/lowering_handle.cpp
+++ b/toolchain/lowering/lowering_handle.cpp
@@ -2,6 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
+#include "llvm/ADT/STLExtras.h"
+#include "llvm/ADT/Sequence.h"
 #include "toolchain/lowering/lowering_function_context.h"
 #include "toolchain/semantics/semantics_node_kind.h"
 
@@ -63,7 +65,7 @@ auto LoweringHandleArrayValue(LoweringFunctionContext& context,
   auto* tuple_type =
       context.GetType(context.semantics_ir().GetNode(tuple_node_id).type_id());
 
-  for (int i = 0; i < static_cast<int>(llvm_type->getArrayNumElements()); ++i) {
+  for (auto i : llvm::seq(llvm_type->getArrayNumElements())) {
     llvm::Value* array_element_value = context.GetIndexFromStructOrArray(
         tuple_type, tuple_value, i, "array.element");
     if (tuple_value->getType()->isPointerTy()) {
@@ -306,14 +308,9 @@ auto LoweringHandleTupleValue(LoweringFunctionContext& context,
       context.builder().CreateAlloca(llvm_type, /*ArraySize=*/nullptr, "tuple");
   context.SetLocal(node_id, alloca);
   auto refs = context.semantics_ir().GetNodeBlock(node.GetAsTupleValue());
-  auto type_refs = context.semantics_ir().GetTypeBlock(
-      context.semantics_ir()
-          .GetNode(context.semantics_ir().GetType(node.type_id()))
-          .GetAsTupleType());
-
-  for (int i = 0; i < static_cast<int>(type_refs.size()); ++i) {
+  for (auto [i, ref] : llvm::enumerate(refs)) {
     auto* gep = context.builder().CreateStructGEP(llvm_type, alloca, i);
-    context.builder().CreateStore(context.GetLocal(refs[i]), gep);
+    context.builder().CreateStore(context.GetLocal(ref), gep);
   }
 }
 
@@ -337,13 +334,13 @@ auto LoweringHandleStructValue(LoweringFunctionContext& context,
       context.semantics_ir()
           .GetNode(context.semantics_ir().GetType(node.type_id()))
           .GetAsStructType());
-  for (int i = 0; i < static_cast<int>(refs.size()); ++i) {
+  for (auto [i, ref, type_ref] : llvm::enumerate(refs, type_refs)) {
     auto [field_name_id, field_type_id] =
-        context.semantics_ir().GetNode(type_refs[i]).GetAsStructTypeField();
+        context.semantics_ir().GetNode(type_ref).GetAsStructTypeField();
     auto member_name = context.semantics_ir().GetString(field_name_id);
     auto* gep =
         context.builder().CreateStructGEP(llvm_type, alloca, i, member_name);
-    context.builder().CreateStore(context.GetLocal(refs[i]), gep);
+    context.builder().CreateStore(context.GetLocal(ref), gep);
   }
 }
 
diff --git a/toolchain/parser/parse_tree.cpp b/toolchain/parser/parse_tree.cpp
index 86638b2f0e93..2bc3ce589dee 100644
--- a/toolchain/parser/parse_tree.cpp
+++ b/toolchain/parser/parse_tree.cpp
@@ -4,9 +4,6 @@
 
 #include "toolchain/parser/parse_tree.h"
 
-#include <cstdlib>
-#include <optional>
-
 #include "common/check.h"
 #include "common/error.h"
 #include "llvm/ADT/Sequence.h"
@@ -244,7 +241,7 @@ auto ParseTree::Verify() const -> ErrorOr<Success> {
         }
       }
     } else {
-      for (int i = 0; i < n_impl.kind.child_count(); ++i) {
+      for (int i : llvm::seq(n_impl.kind.child_count())) {
         if (nodes.empty()) {
           return Error(llvm::formatv(
               "Node #{0} is a {1} with child_count {2}, but only had {3} "
diff --git a/toolchain/parser/parser_context.cpp b/toolchain/parser/parser_context.cpp
index 54bc85ded1c8..899a4b22096a 100644
--- a/toolchain/parser/parser_context.cpp
+++ b/toolchain/parser/parser_context.cpp
@@ -4,11 +4,10 @@
 
 #include "toolchain/parser/parser_context.h"
 
-#include <cstdlib>
-#include <memory>
 #include <optional>
 
 #include "common/check.h"
+#include "llvm/ADT/STLExtras.h"
 #include "toolchain/lexer/token_kind.h"
 #include "toolchain/lexer/tokenized_buffer.h"
 #include "toolchain/parser/parse_node_kind.h"
@@ -448,8 +447,7 @@ auto ParserContext::EmitExpectedDeclarationSemiOrDefinition(
 
 auto ParserContext::PrintForStackDump(llvm::raw_ostream& output) const -> void {
   output << "Parser stack:\n";
-  for (int i = 0; i < static_cast<int>(state_stack_.size()); ++i) {
-    const auto& entry = state_stack_[i];
+  for (auto [i, entry] : llvm::enumerate(state_stack_)) {
     output << "\t" << i << ".\t" << entry.state;
     PrintTokenForStackDump(output, entry.token);
   }
diff --git a/toolchain/semantics/semantics_context.cpp b/toolchain/semantics/semantics_context.cpp
index 8b5a134ae137..58553524e8c0 100644
--- a/toolchain/semantics/semantics_context.cpp
+++ b/toolchain/semantics/semantics_context.cpp
@@ -8,6 +8,7 @@
 
 #include "common/check.h"
 #include "common/vlog.h"
+#include "llvm/ADT/STLExtras.h"
 #include "toolchain/diagnostics/diagnostic_kind.h"
 #include "toolchain/lexer/tokenized_buffer.h"
 #include "toolchain/parser/parse_node_kind.h"
@@ -298,9 +299,8 @@ auto SemanticsContext::ImplicitAsForArgs(
   // Check type conversions per-element.
   // TODO: arg_ir_id is passed so that implicit conversions can be inserted.
   // It's currently not supported, but will be needed.
-  for (size_t i = 0; i < arg_refs.size(); ++i) {
-    auto value_id = arg_refs[i];
-    auto as_type_id = semantics_ir_->GetNode(param_refs[i]).type_id();
+  for (auto [i, value_id, param_ref] : llvm::enumerate(arg_refs, param_refs)) {
+    auto as_type_id = semantics_ir_->GetNode(param_ref).type_id();
     if (ImplicitAsImpl(value_id, as_type_id,
                        diagnostic == nullptr ? &value_id : nullptr) ==
         ImplicitAsKind::Incompatible) {
diff --git a/toolchain/semantics/semantics_handle_name.cpp b/toolchain/semantics/semantics_handle_name.cpp
index 3f0ea604003f..7c2f62792fc0 100644
--- a/toolchain/semantics/semantics_handle_name.cpp
+++ b/toolchain/semantics/semantics_handle_name.cpp
@@ -2,6 +2,7 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
+#include "llvm/ADT/STLExtras.h"
 #include "toolchain/semantics/semantics_context.h"
 #include "toolchain/semantics/semantics_node.h"
 
@@ -30,8 +31,8 @@ auto SemanticsHandleMemberAccessExpression(SemanticsContext& context,
       auto refs =
           context.semantics_ir().GetNodeBlock(base_type.GetAsStructType());
       // TODO: Do we need to optimize this with a lookup table for O(1)?
-      for (int i = 0; i < static_cast<int>(refs.size()); ++i) {
-        auto ref = context.semantics_ir().GetNode(refs[i]);
+      for (auto [i, ref_id] : llvm::enumerate(refs)) {
+        auto ref = context.semantics_ir().GetNode(ref_id);
         if (auto [field_name_id, field_type_id] = ref.GetAsStructTypeField();
             name_id == field_name_id) {
           context.AddNodeAndPush(
diff --git a/toolchain/semantics/semantics_ir.cpp b/toolchain/semantics/semantics_ir.cpp
index b09afd57a644..7af85a605af9 100644
--- a/toolchain/semantics/semantics_ir.cpp
+++ b/toolchain/semantics/semantics_ir.cpp
@@ -5,9 +5,8 @@
 #include "toolchain/semantics/semantics_ir.h"
 
 #include "common/check.h"
-#include "llvm/ADT/DenseSet.h"
+#include "llvm/ADT/Sequence.h"
 #include "llvm/ADT/SmallVector.h"
-#include "llvm/Support/SaveAndRestore.h"
 #include "toolchain/base/pretty_stack_trace_function.h"
 #include "toolchain/parser/parse_tree_node_location_translator.h"
 #include "toolchain/semantics/semantics_builtin_kind.h"
@@ -52,7 +51,7 @@ auto SemanticsIR::MakeFromParseTree(const SemanticsIR& builtin_ir,
   // Copy builtins over.
   semantics_ir.nodes_.resize_for_overwrite(SemanticsBuiltinKind::ValidCount);
   static constexpr auto BuiltinIR = SemanticsCrossReferenceIRId(0);
-  for (int i = 0; i < SemanticsBuiltinKind::ValidCount; ++i) {
+  for (int i : llvm::seq(SemanticsBuiltinKind::ValidCount)) {
     // We can reuse the type node ID because the offsets of cross-references
     // will be the same in this IR.
     auto type = builtin_ir.nodes_[i].type_id();
diff --git a/toolchain/semantics/semantics_ir_formatter.cpp b/toolchain/semantics/semantics_ir_formatter.cpp
index 89bb25038f6d..45aa59e98df6 100644
--- a/toolchain/semantics/semantics_ir_formatter.cpp
+++ b/toolchain/semantics/semantics_ir_formatter.cpp
@@ -4,8 +4,8 @@
 
 #include "toolchain/semantics/semantics_ir_formatter.h"
 
+#include "llvm/ADT/Sequence.h"
 #include "llvm/ADT/StringExtras.h"
-#include "llvm/ADT/StringSet.h"
 #include "llvm/Support/SaveAndRestore.h"
 #include "toolchain/lexer/tokenized_buffer.h"
 #include "toolchain/parser/parse_tree.h"
@@ -45,7 +45,7 @@ class NodeNamer {
     CollectNamesInBlock(ScopeIndex::Package, semantics_ir.top_node_block_id());
 
     // Build each function scope.
-    for (int i = 0; i != semantics_ir.functions_size(); ++i) {
+    for (int i : llvm::seq(semantics_ir.functions_size())) {
       auto fn_id = SemanticsFunctionId(i);
       auto fn_scope = GetScopeFor(fn_id);
       const auto& fn = semantics_ir.GetFunction(fn_id);
@@ -405,7 +405,7 @@ class SemanticsIRFormatter {
     }
     out_ << "}\n";
 
-    for (int i = 0; i != semantics_ir_.functions_size(); ++i) {
+    for (int i : llvm::seq(semantics_ir_.functions_size())) {
       FormatFunction(SemanticsFunctionId(i));
     }
   }
diff --git a/toolchain/semantics/semantics_node_block_stack.cpp b/toolchain/semantics/semantics_node_block_stack.cpp
index 7a8e75dfbee6..361d491aa941 100644
--- a/toolchain/semantics/semantics_node_block_stack.cpp
+++ b/toolchain/semantics/semantics_node_block_stack.cpp
@@ -5,6 +5,7 @@
 #include "toolchain/semantics/semantics_node_block_stack.h"
 
 #include "common/vlog.h"
+#include "llvm/ADT/STLExtras.h"
 #include "toolchain/semantics/semantics_node.h"
 
 namespace Carbon {
@@ -40,8 +41,8 @@ auto SemanticsNodeBlockStack::Pop() -> SemanticsNodeBlockId {
 auto SemanticsNodeBlockStack::PrintForStackDump(llvm::raw_ostream& output) const
     -> void {
   output << name_ << ":\n";
-  for (int i = 0; i < static_cast<int>(stack_.size()); ++i) {
-    output << "\t" << i << ".\t" << stack_[i] << "\n";
+  for (auto [i, entry] : llvm::enumerate(stack_)) {
+    output << "\t" << i << ".\t" << entry << "\n";
   }
 }
 
diff --git a/toolchain/semantics/semantics_node_stack.cpp b/toolchain/semantics/semantics_node_stack.cpp
index f2fbf7b1dd07..72b315df5200 100644
--- a/toolchain/semantics/semantics_node_stack.cpp
+++ b/toolchain/semantics/semantics_node_stack.cpp
@@ -4,6 +4,7 @@
 
 #include "toolchain/semantics/semantics_node_stack.h"
 
+#include "llvm/ADT/STLExtras.h"
 #include "toolchain/semantics/semantics_node.h"
 
 namespace Carbon {
@@ -11,8 +12,7 @@ namespace Carbon {
 auto SemanticsNodeStack::PrintForStackDump(llvm::raw_ostream& output) const
     -> void {
   output << "SemanticsNodeStack:\n";
-  for (int i = 0; i < static_cast<int>(stack_.size()); ++i) {
-    const auto& entry = stack_[i];
+  for (auto [i, entry] : llvm::enumerate(stack_)) {
     auto parse_node_kind = parse_tree_->node_kind(entry.parse_node);
     output << "\t" << i << ".\t" << parse_node_kind;
     if (parse_node_kind == ParseNodeKind::PatternBinding) {