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
@@ -58,6 +58,7 @@
   unsigned HashOffset = 0; // Byte offset from start of file to #.
   int HashLine = 0;        // Line number containing the directive, 0-indexed.
   SrcMgr::CharacteristicKind FileKind = SrcMgr::C_User;
+  bool Used = true; // Was something from this used in the main file?
 };
 llvm::raw_ostream &operator<<(llvm::raw_ostream &, const Inclusion &);
 bool operator==(const Inclusion &LHS, const Inclusion &RHS);
@@ -129,6 +130,27 @@
                      llvm::StringRef IncludedName,
                      llvm::StringRef IncludedRealName);
 
+  // Captures the filename corresponding to system includes like <vector>.
+  // This is used in for IWYU of the standard library as we want to check
+  // against where the spec *says* the symbols are.
+  void recordSystemInclude(llvm::StringRef Spelled,
+                           llvm::StringRef IncludedName);
+
+  // Classifying the main-file includes as "used" or "unused" is subtle
+  // (consider transitive includes), so we inject the algorithm.
+
+  // An AbstractIncludeGraph maps including files to included files.
+  using AbstractIncludeGraph = llvm::DenseMap<unsigned, SmallVector<unsigned>>;
+  // A UsedFunc decides whether each file included by EntryPoint is "used",
+  // based on the set of files that contain some referenced symbol.
+  using UsedFunc = llvm::DenseMap<unsigned, bool>(
+      const AbstractIncludeGraph &, llvm::DenseSet<unsigned> Referenced,
+      unsigned EntryPoint);
+  // Mark each MainFileIncludes with the decision yielded by the UsedFunc.
+  void markUsed(llvm::StringRef EntryPoint,
+                llvm::ArrayRef<StringRef> ReferencedFiles,
+                llvm::function_ref<UsedFunc>);
+
 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/Headers.cpp b/clang-tools-extra/clangd/Headers.cpp
--- a/clang-tools-extra/clangd/Headers.cpp
+++ b/clang-tools-extra/clangd/Headers.cpp
@@ -102,6 +102,18 @@
   IncludeStructure *Out;
 };
 
+// Map of unquoted to quoted
+const llvm::StringMap<llvm::StringRef> &allStandardIncludes() {
+  static llvm::StringMap<llvm::StringRef> Result = {
+#define SYMBOL(Name, Namespace, Header)                                        \
+  {llvm::StringRef(#Header).ltrim('<').rtrim('>'), #Header},
+#include "CSymbolMap.inc"
+#include "StdSymbolMap.inc"
+#undef SYMBOL
+  };
+  return Result;
+}
+
 } // namespace
 
 bool isLiteralInclude(llvm::StringRef Include) {
@@ -204,6 +216,46 @@
   return Result;
 }
 
+void IncludeStructure::markUsed(llvm::StringRef EntryPoint,
+                                llvm::ArrayRef<StringRef> ReferencedFiles,
+                                llvm::function_ref<UsedFunc> Algorithm) {
+  auto Root = NameToIndex.find(EntryPoint);
+  if (Root == NameToIndex.end()) {
+    elog("IWYU: entrypoint {0} not found in include graph", EntryPoint);
+    return;
+  }
+
+  llvm::DenseSet<unsigned> Referenced;
+  Referenced.reserve(ReferencedFiles.size());
+  for (llvm::StringRef RefName : ReferencedFiles) {
+    dlog("{0} is REFERENCED", RefName);
+    auto It = NameToIndex.find(RefName);
+    if (It != NameToIndex.end())
+      Referenced.insert(It->second);
+  }
+
+  auto Decisions =
+      Algorithm(IncludeChildren, std::move(Referenced), Root->second);
+  auto RootChildren = IncludeChildren.find(Root->second);
+  assert(RootChildren != IncludeChildren.end());
+  llvm::DenseMap</*RealPath*/ StringRef, /*Index*/ unsigned> IncludeIndex;
+  for (const auto Index : RootChildren->second) {
+    if (!RealPathNames[Index].empty())
+      IncludeIndex[RealPathNames[Index]] = Index;
+  }
+  for (auto &MFI : MainFileIncludes) {
+    // FIXME: Skip includes that are not self-contained.
+    auto It = IncludeIndex.find(MFI.Resolved);
+    if (It != IncludeIndex.end()) {
+      auto DIt = Decisions.find(It->second);
+      if (DIt != Decisions.end()) {
+        MFI.Used = DIt->second;
+        dlog("{0} is {1}", MFI.Written, MFI.Used ? "USED" : "UNUSED");
+      }
+    }
+  }
+}
+
 void IncludeInserter::addExisting(const Inclusion &Inc) {
   IncludedHeaders.insert(Inc.Written);
   if (!Inc.Resolved.empty())
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 +1,175 @@
+//===--- 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;
+  }
+
+#if 0
+  // 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.
+  bool VisitExpr(Expr *E) {
+    TraverseType(E->getType());
+    return true;
+  }
+#endif
+
+  bool TraverseType(QualType T) {
+    if (shouldAdd(T.getTypePtrOrNull())) { // don't care about quals
+      Base::TraverseType(T);
+    }
+    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/ParsedAST.h b/clang-tools-extra/clangd/ParsedAST.h
--- a/clang-tools-extra/clangd/ParsedAST.h
+++ b/clang-tools-extra/clangd/ParsedAST.h
@@ -83,6 +83,8 @@
     return getASTContext().getLangOpts();
   }
 
+  void computeUsedIncludes();
+
   /// This function returns top-level decls present in the main file of the AST.
   /// The result does not include the decls that come from the preamble.
   /// (These should be const, but RecursiveASTVisitor requires Decl*).
diff --git a/clang-tools-extra/clangd/ParsedAST.cpp b/clang-tools-extra/clangd/ParsedAST.cpp
--- a/clang-tools-extra/clangd/ParsedAST.cpp
+++ b/clang-tools-extra/clangd/ParsedAST.cpp
@@ -18,6 +18,7 @@
 #include "Features.h"
 #include "Headers.h"
 #include "HeuristicResolver.h"
+#include "IWYU.h"
 #include "IncludeFixer.h"
 #include "Preamble.h"
 #include "SourceCode.h"
@@ -601,5 +602,26 @@
     return llvm::None;
   return llvm::StringRef(Preamble->Version);
 }
+
+void ParsedAST::computeUsedIncludes() {
+  const auto &SM = getSourceManager();
+
+  auto Refs = findReferencedLocations(*this);
+  auto ReferencedFileIDs =
+      findReferencedFiles(Refs.AllLocations, getSourceManager());
+  std::vector<llvm::StringRef> ReferencedFilenames;
+  ReferencedFilenames.reserve(ReferencedFileIDs.size());
+  for (FileID FID : ReferencedFileIDs) {
+    const FileEntry *FE = SM.getFileEntryForID(FID);
+    if (!FE) {
+      elog("Missing FE for {0}", SM.getComposedLoc(FID, 0).printToString(SM));
+      continue;
+    }
+    ReferencedFilenames.push_back(SM.getFileEntryForID(FID)->getName());
+  }
+  Includes.markUsed(SM.getFileEntryForID(SM.getMainFileID())->getName(),
+                    ReferencedFilenames, directlyReferencedFiles);
+}
+
 } // 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 +1,98 @@
+//===--- 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;",
+      },
+      {
+          "template <typename> class ^X;",
+          "X<int> *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();",
+      },
+      {
+          "typedef bool ^Y; template <typename T> struct ^X {};",
+          "X<Y> x;",
+      },
+      {
+          // 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