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/IWYU.h b/clang-tools-extra/clangd/IWYU.h
new file mode 100644
--- /dev/null
+++ b/clang-tools-extra/clangd/IWYU.h
@@ -0,0 +1,55 @@
+//===--- 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 {
+
+using ReferencedLocations = llvm::DenseSet<SourceLocation>;
+/// Goes through main file AST and computes all the locations used symbols are
+/// coming from. These locations are later used to determine which headers
+/// should be marked as "used" and "directly used".
+///
+/// Because of the clangd limitations, this should be efficient,
+/// hence there is only one AST traversal for the whole process.
+///
+/// NOTE: The balance is currently shifted towards considering some of the
+/// implicitly used symbols as "used" for the sake of providing less noisy
+/// "unused header" diagnostics. However, this should be re-evalated once we
+/// also want to surface "missing header" diagnostics.
+ReferencedLocations findReferencedLocations(ParsedAST &AST);
+
+/// Computes all referenced files given a set of SourceLocations that were
+/// extracted from symbol declarations.
+llvm::DenseSet<FileID> findReferencedFiles(const ReferencedLocations &Locs,
+                                           const SourceManager &SM);
+
+} // 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 +1,158 @@
+//===--- 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 {
+
+/// Main "driver" of the IWYU symbol usage discovery mechanism. Crawler
+/// traverses the AST and feeds in the locations of (sometimes implicitly) used
+/// symbols into \p Result.
+class ReferencedLocationCrawler
+    : public RecursiveASTVisitor<ReferencedLocationCrawler> {
+public:
+  ReferencedLocationCrawler(ReferencedLocations &Result) : Result(Result) {}
+
+  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
+      RecursiveASTVisitor<ReferencedLocationCrawler>::TraverseType(T);
+    }
+    return true;
+  }
+
+  bool VisitUsingDecl(UsingDecl *D) {
+    for (const auto *Shadow : D->shadows()) {
+      add(Shadow->getTargetDecl());
+    }
+    return true;
+  }
+
+private:
+  void add(const Decl *D) {
+    if (!D || !shouldAdd(D->getCanonicalDecl())) {
+      return;
+    }
+    for (const Decl *Redecl : D->redecls()) {
+      Result.insert(Redecl->getLocation());
+    }
+  }
+
+  bool shouldAdd(const void *P) { return P && Visited.insert(P).second; }
+
+  ReferencedLocations &Result;
+  llvm::DenseSet<const void *> Visited;
+};
+
+// 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 Crawler(Result);
+  Crawler.TraverseAST(AST.getASTContext());
+  // FIXME(kirillbobyrev): Handle macros.
+  return Result;
+}
+
+llvm::DenseSet<FileID> findReferencedFiles(const ReferencedLocations &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();) {
+    auto 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 +1,127 @@
+//===--- 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;",
+      },
+      {
+          "namespace ns { struct X; struct X {}; }",
+          "using namespace ns;",
+      },
+      {
+          // FIXME(kirillbobyrev): Should B also be marked as used?
+          "struct ^A {}; using B = A; using ^C = B;",
+          "C a;",
+      },
+      {
+          "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)) {
+      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