diff --git a/clang-tools-extra/clangd/CMakeLists.txt b/clang-tools-extra/clangd/CMakeLists.txt
--- a/clang-tools-extra/clangd/CMakeLists.txt
+++ b/clang-tools-extra/clangd/CMakeLists.txt
@@ -85,6 +85,7 @@
   Hover.cpp
   IncludeFixer.cpp
   InlayHints.cpp
+  IWYU.cpp
   JSONTransport.cpp
   PathMapping.cpp
   Protocol.cpp
diff --git a/clang-tools-extra/clangd/Headers.h b/clang-tools-extra/clangd/Headers.h
--- a/clang-tools-extra/clangd/Headers.h
+++ b/clang-tools-extra/clangd/Headers.h
@@ -129,6 +129,8 @@
                      llvm::StringRef IncludedName,
                      llvm::StringRef IncludedRealName);
 
+  using AbstractIncludeGraph = llvm::DenseMap<unsigned, SmallVector<unsigned>>;
+
 private:
   // Identifying files in a way that persists from preamble build to subsequent
   // builds is surprisingly hard. FileID is unavailable in InclusionDirective(),
diff --git a/clang-tools-extra/clangd/IWYU.h b/clang-tools-extra/clangd/IWYU.h
new file mode 100644
--- /dev/null
+++ b/clang-tools-extra/clangd/IWYU.h
@@ -0,0 +1,60 @@
+//===--- IWYU.h - include-what-you-use analysis -----------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// This defines functionality modeled after Include-What-You-Use
+/// (https://include-what-you-use.org/). The variant we implement is not
+/// equivalent to the original tool since we have different design limitations
+/// such as performance requirements but we are trying to make it compatible
+/// with the original tool in the most popular and useful cases.
+///
+/// FIXME(kirillbobyrev): Add support for IWYU pragmas.
+/// FIXME(kirillbobyrev): Add support for mapping files.
+/// FIXME(kirillbobyrev): Add support for standard library headers.
+///
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_IWYU_H
+#define LLVM_CLANG_TOOLS_EXTRA_CLANGD_IWYU_H
+
+#include "Headers.h"
+#include "ParsedAST.h"
+#include "clang/Basic/SourceLocation.h"
+#include "llvm/ADT/DenseSet.h"
+
+namespace clang {
+namespace clangd {
+
+struct ReferencedLocations {
+  llvm::DenseSet<SourceLocation> AllLocations;
+};
+ReferencedLocations findReferencedLocations(ParsedAST &AST);
+
+/// Computes all referenced files given a set of SourceLocations that were
+/// extracted from symbol declarations.
+llvm::DenseSet<FileID>
+findReferencedFiles(const llvm::DenseSet<SourceLocation> &Locs,
+                    const SourceManager &SM);
+
+/// Given an include graph, computes files that are referenced directly from the
+/// main file (the one where diagnostics will go to, identified by passed in
+/// \p EntryPoint).
+inline llvm::DenseMap<unsigned, bool>
+directlyReferencedFiles(const IncludeStructure::AbstractIncludeGraph &Graph,
+                        const llvm::DenseSet<unsigned> &Referenced,
+                        unsigned EntryPoint) {
+  llvm::DenseMap<unsigned, bool> Result;
+  for (unsigned Inclusion : Graph.lookup(EntryPoint))
+    Result.try_emplace(Inclusion, Referenced.contains(Inclusion));
+  return Result;
+}
+
+} // namespace clangd
+} // namespace clang
+
+#endif
diff --git a/clang-tools-extra/clangd/IWYU.cpp b/clang-tools-extra/clangd/IWYU.cpp
new file mode 100644
--- /dev/null
+++ b/clang-tools-extra/clangd/IWYU.cpp
@@ -0,0 +1,183 @@
+//===--- IWYU.cpp -----------------------------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "IWYU.h"
+#include "support/Logger.h"
+#include "clang/AST/RecursiveASTVisitor.h"
+#include "clang/Basic/SourceLocation.h"
+
+namespace clang {
+namespace clangd {
+namespace {
+
+// FIXME(kirillbobyrev): We do not handle symbols from the standard library yet.
+bool inStdNamespace(const DeclContext *DC, std::string &Buf) {
+  const NamespaceDecl *LeadingNamespace;
+  unsigned NamespaceCount = 0;
+  for (; !DC->isTranslationUnit(); DC = DC->getParent()) {
+    if (!DC->isTransparentContext()) {
+      if (DC->isNamespace()) {
+        if (!NamespaceCount)
+          LeadingNamespace = cast<NamespaceDecl>(DC);
+        ++NamespaceCount;
+      } else {
+        return false;
+      }
+    }
+  }
+  if (!NamespaceCount)
+    return false;
+  if (NamespaceCount == 1)
+    return LeadingNamespace->getName().startswith("std");
+  Buf = printNamespaceScope(*LeadingNamespace);
+  return llvm::StringRef(Buf).rtrim(":").startswith("std");
+}
+
+class ReferencedLocationCrawler
+    : public RecursiveASTVisitor<ReferencedLocationCrawler> {
+  using Base = RecursiveASTVisitor<ReferencedLocationCrawler>;
+  llvm::DenseSet<const void *> Visited;
+  bool shouldAdd(const void *P) { return P && Visited.insert(P).second; }
+
+  void add(const Decl *D) {
+    std::string Buffer;
+    if (!D || inStdNamespace(D->getDeclContext(), Buffer) ||
+        !shouldAdd(D->getCanonicalDecl())) {
+      return;
+    }
+    // FIXME(kirillbobyrev): Should we only use the main declaration instead?
+    for (const Decl *Redecl : D->redecls()) {
+      Result.AllLocations.insert(Redecl->getLocation());
+    }
+  }
+
+public:
+  bool VisitDeclRefExpr(DeclRefExpr *DRE) {
+    add(DRE->getDecl());
+    add(DRE->getFoundDecl());
+    return true;
+  }
+
+  bool VisitMemberExpr(MemberExpr *ME) {
+    add(ME->getMemberDecl());
+    add(ME->getFoundDecl().getDecl());
+    return true;
+  }
+
+  bool VisitTagType(TagType *TT) {
+    add(TT->getDecl());
+    return true;
+  }
+
+  bool VisitCXXConstructExpr(CXXConstructExpr *CCE) {
+    add(CCE->getConstructor());
+    return true;
+  }
+
+  bool VisitTemplateSpecializationType(TemplateSpecializationType *TST) {
+    if (shouldAdd(TST)) {
+      add(TST->getTemplateName().getAsTemplateDecl()); // primary template
+      add(TST->getAsCXXRecordDecl());                  // specialization
+    }
+    return true;
+  }
+
+  bool VisitTypedefType(TypedefType *TT) {
+    add(TT->getDecl());
+    return true;
+  }
+
+  // Consider types of any subexpression used, even if the type is not named.
+  // This is helpful in getFoo().bar(), where Foo must be complete.
+  // FIXME(kirillbobyrev): This is a tricky case that raises a question - should
+  // we consider implicit types to be "used"? With auto's it's not so clear.
+  // This should probably be controlled with a flag.
+  bool VisitExpr(Expr *E) {
+    TraverseType(E->getType());
+    return true;
+  }
+
+  bool TraverseType(QualType T) {
+    if (shouldAdd(T.getTypePtrOrNull())) { // don't care about quals
+      Base::TraverseType(T);
+    }
+    return true;
+  }
+
+  bool VisitUsingDecl(UsingDecl *D) {
+    for (const auto *Shadow : D->shadows()) {
+      if (shouldAdd(Shadow->getTargetDecl())) {
+        add(Shadow->getTargetDecl());
+      }
+    }
+    return true;
+  }
+
+  ReferencedLocationCrawler(ReferencedLocations &Result) : Result(Result){};
+  ReferencedLocations &Result;
+};
+
+// Given a set of referenced FileIDs, determines all the potentially-referenced
+// files and macros by traversing expansion/spelling locations of macro IDs.
+// This is used to map the referenced SourceLocations onto real files.
+struct ReferencedFiles {
+  ReferencedFiles(const SourceManager &SM) : SM(SM) {}
+  llvm::DenseSet<FileID> Files;
+  llvm::DenseSet<FileID> Macros;
+  const SourceManager &SM;
+
+  void add(SourceLocation Loc) { add(SM.getFileID(Loc), Loc); }
+
+  void add(FileID FID, SourceLocation Loc) {
+    if (FID.isInvalid())
+      return;
+    assert(SM.isInFileID(Loc, FID));
+    if (Loc.isFileID()) {
+      Files.insert(FID);
+      return;
+    }
+    // Don't process the same macro FID twice.
+    if (!Macros.insert(FID).second)
+      return;
+    const auto &Exp = SM.getSLocEntry(FID).getExpansion();
+    add(Exp.getSpellingLoc());
+    add(Exp.getExpansionLocStart());
+    add(Exp.getExpansionLocEnd());
+  }
+};
+
+} // namespace
+
+ReferencedLocations findReferencedLocations(ParsedAST &AST) {
+  ReferencedLocations Result;
+  ReferencedLocationCrawler C(Result);
+  C.TraverseAST(AST.getASTContext());
+  // FIXME(kirillbobyrev): Handle macros.
+  return Result;
+}
+
+llvm::DenseSet<FileID>
+findReferencedFiles(const llvm::DenseSet<SourceLocation> &Locs,
+                    const SourceManager &SM) {
+  std::vector<SourceLocation> Sorted{Locs.begin(), Locs.end()};
+  llvm::sort(Sorted); // This groups by FileID!
+  ReferencedFiles Result(SM);
+  for (auto It = Sorted.begin(); It < Sorted.end();) {
+    FileID FID = SM.getFileID(*It);
+    Result.add(FID, *It);
+    // Cheaply skip over all the other locations from the same FileID.
+    // This avoids lots of redundant Loc->File lookups for the same file.
+    do {
+      ++It;
+    } while (It != Sorted.end() && SM.isInFileID(*It, FID));
+  }
+  return std::move(Result.Files);
+}
+
+} // namespace clangd
+} // namespace clang
diff --git a/clang-tools-extra/clangd/unittests/CMakeLists.txt b/clang-tools-extra/clangd/unittests/CMakeLists.txt
--- a/clang-tools-extra/clangd/unittests/CMakeLists.txt
+++ b/clang-tools-extra/clangd/unittests/CMakeLists.txt
@@ -61,6 +61,7 @@
   IndexActionTests.cpp
   IndexTests.cpp
   InlayHintTests.cpp
+  IWYUTests.cpp
   JSONTransportTests.cpp
   LoggerTests.cpp
   LSPBinderTests.cpp
diff --git a/clang-tools-extra/clangd/unittests/IWYUTests.cpp b/clang-tools-extra/clangd/unittests/IWYUTests.cpp
new file mode 100644
--- /dev/null
+++ b/clang-tools-extra/clangd/unittests/IWYUTests.cpp
@@ -0,0 +1,118 @@
+//===--- IWYUTests.cpp ------------------------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "Annotations.h"
+#include "IWYU.h"
+#include "TestTU.h"
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+
+namespace clang {
+namespace clangd {
+namespace {
+
+using testing::ElementsAreArray;
+
+TEST(IWYU, ReferencedLocations) {
+  struct TestCase {
+    std::string HeaderCode;
+    std::string MainCode;
+  };
+  TestCase Cases[] = {
+      {
+          "int ^x();",
+          "int y = x();",
+      },
+      {
+          "class ^X;",
+          "X *y;",
+      },
+      {
+          "using ^Integer = int;",
+          "Integer x;",
+      },
+      {
+          "struct ^X{int ^a;}; X ^foo();",
+          "int y = foo().a;",
+      },
+      {
+          "class ^X{}; X ^foo();",
+          "auto bar() { return foo(); }",
+      },
+      {
+          "class ^X; class ^X{}; class ^X;",
+          "X *y;",
+      },
+      {
+          "struct ^X { ^X(int) {} int ^foo(); };",
+          "auto x = X(42); auto y = x.foo();",
+      },
+      {
+          "struct ^X { static bool ^foo(); }; bool X::^foo() {}",
+          "auto b = X::foo();",
+      },
+      {
+          "template <typename> class ^X;",
+          "X<int> *y;",
+      },
+      {
+          "typedef bool ^Y; template <typename T> struct ^X {};",
+          "X<Y> x;",
+      },
+      {
+          "struct Foo; struct ^Foo{}; typedef Foo ^Bar;",
+          "Bar b;",
+      },
+      {
+          "class ^X{}; X ^getX();",
+          "auto x = getX();",
+      },
+      {
+          "namespace ns { struct ^X; struct ^X {}; }",
+          "using ns::X;",
+      },
+      {
+          "enum ^Color { ^Red = 42, Green = 9000};",
+          "int MyColor = Red;",
+      },
+      {
+          "struct ^X { enum ^Language { ^CXX = 42, Python = 9000}; };",
+          "int Lang = X::CXX;",
+      },
+      {
+          // FIXME(kirillbobyrev): Should report the UsingDecl.
+          // This information is not preserved in the AST.
+          //                                 ^
+          "namespace ns { class ^X; }; using ns::X;",
+          "X *y;",
+      }};
+  for (const TestCase &T : Cases) {
+    TestTU TU;
+    TU.Code = T.MainCode;
+    Annotations Header(T.HeaderCode);
+    TU.HeaderCode = Header.code().str();
+    auto AST = TU.build();
+
+    std::vector<Position> Points;
+    for (const auto &Loc : findReferencedLocations(AST).AllLocations) {
+      if (AST.getSourceManager().getBufferName(Loc).endswith(
+              TU.HeaderFilename)) {
+        Points.push_back(offsetToPosition(
+            TU.HeaderCode, AST.getSourceManager().getFileOffset(Loc)));
+      }
+    }
+    llvm::sort(Points);
+
+    EXPECT_EQ(Points, Header.points()) << T.HeaderCode << "\n---\n"
+                                       << T.MainCode;
+  }
+}
+
+} // namespace
+} // namespace clangd
+} // namespace clang