diff --git a/clang-tools-extra/clangd/refactor/tweaks/DefineOutline.cpp b/clang-tools-extra/clangd/refactor/tweaks/DefineOutline.cpp
--- a/clang-tools-extra/clangd/refactor/tweaks/DefineOutline.cpp
+++ b/clang-tools-extra/clangd/refactor/tweaks/DefineOutline.cpp
@@ -13,9 +13,17 @@
 #include "refactor/Tweak.h"
 #include "clang/AST/ASTTypeTraits.h"
 #include "clang/AST/Decl.h"
+#include "clang/AST/DeclTemplate.h"
 #include "clang/AST/Stmt.h"
+#include "clang/Basic/SourceLocation.h"
+#include "clang/Basic/SourceManager.h"
+#include "clang/Driver/Types.h"
+#include "clang/Format/Format.h"
+#include "clang/Tooling/Core/Replacement.h"
 #include "llvm/ADT/Optional.h"
-#include "llvm/Support/Path.h"
+#include "llvm/ADT/StringRef.h"
+#include "llvm/Support/Error.h"
+#include <cstddef>
 
 namespace clang {
 namespace clangd {
@@ -40,6 +48,51 @@
   return nullptr;
 }
 
+llvm::Optional<Path> getSourceFile(llvm::StringRef FileName,
+                                   const Tweak::Selection &Sel) {
+  if (auto Source = getCorrespondingHeaderOrSource(
+          FileName,
+          &Sel.AST.getSourceManager().getFileManager().getVirtualFileSystem()))
+    return *Source;
+  return getCorrespondingHeaderOrSource(FileName, Sel.AST, Sel.Index);
+}
+
+// Creates a modified version of function definition that can be inserted at a
+// different location. Contains both function signature and body.
+llvm::Optional<llvm::StringRef> getFunctionSourceCode(const FunctionDecl *FD) {
+  auto &SM = FD->getASTContext().getSourceManager();
+  auto CharRange = toHalfOpenFileRange(SM, FD->getASTContext().getLangOpts(),
+                                       FD->getSourceRange());
+  if (!CharRange)
+    return llvm::None;
+  // Include template parameter list.
+  if (auto *FTD = FD->getDescribedFunctionTemplate())
+    CharRange->setBegin(FTD->getBeginLoc());
+
+  // FIXME: Qualify return type.
+  // FIXME: Qualify function name depending on the target context.
+  return toSourceCode(SM, *CharRange);
+}
+
+// Returns the most natural insertion point for \p QualifiedName in \p Contents.
+// This currently cares about only the namespace proximity, but in feature it
+// should also try to follow ordering of declarations. For example, if decls
+// come in order `foo, bar, baz` then this function should return some point
+// between foo and baz for inserting bar.
+llvm::Expected<size_t> getInsertionOffset(llvm::StringRef Contents,
+                                          llvm::StringRef QualifiedName,
+                                          const format::FormatStyle &Style) {
+  auto Region = getEligiblePoints(Contents, QualifiedName, Style);
+
+  assert(!Region.EligiblePoints.empty());
+  // FIXME: This selection can be made smarter by looking at the definition
+  // locations for adjacent decls to Source. Unfortunately psudeo parsing in
+  // getEligibleRegions only knows about namespace begin/end events so we
+  // can't match function start/end positions yet.
+  auto InsertionPoint = Region.EligiblePoints.back();
+  return positionToOffset(Contents, InsertionPoint);
+}
+
 /// Moves definition of a function/method to an appropriate implementation file.
 ///
 /// Before:
@@ -94,8 +147,64 @@
   }
 
   Expected<Effect> apply(const Selection &Sel) override {
-    return llvm::createStringError(llvm::inconvertibleErrorCode(),
-                                   "Not implemented yet");
+    const SourceManager &SM = Sel.AST.getSourceManager();
+    auto MainFileName =
+        getCanonicalPath(SM.getFileEntryForID(SM.getMainFileID()), SM);
+    if (!MainFileName)
+      return llvm::createStringError(
+          llvm::inconvertibleErrorCode(),
+          "Couldn't get absolute path for mainfile.");
+
+    auto CCFile = getSourceFile(*MainFileName, Sel);
+    if (!CCFile)
+      return llvm::createStringError(
+          llvm::inconvertibleErrorCode(),
+          "Couldn't find a suitable implementation file.");
+
+    auto &FS =
+        Sel.AST.getSourceManager().getFileManager().getVirtualFileSystem();
+    auto Buffer = FS.getBufferForFile(*CCFile);
+    // FIXME: Maybe we should consider creating the implementation file if it
+    // doesn't exist?
+    if (!Buffer)
+      return llvm::createStringError(Buffer.getError(),
+                                     Buffer.getError().message());
+    auto Contents = Buffer->get()->getBuffer();
+    auto InsertionOffset =
+        getInsertionOffset(Contents, Source->getQualifiedNameAsString(),
+                           getFormatStyleForFile(*CCFile, Contents, &FS));
+    if (!InsertionOffset)
+      return InsertionOffset.takeError();
+
+    auto FuncDef = getFunctionSourceCode(Source);
+    if (!FuncDef)
+      return llvm::createStringError(
+          llvm::inconvertibleErrorCode(),
+          "Couldn't get full source for function definition.");
+
+    SourceManagerForFile SMFF(*CCFile, Contents);
+    const tooling::Replacement InsertFunctionDef(*CCFile, *InsertionOffset, 0,
+                                                 *FuncDef);
+    auto Effect = Effect::mainFileEdit(
+        SMFF.get(), tooling::Replacements(InsertFunctionDef));
+    if (!Effect)
+      return Effect.takeError();
+
+    // FIXME: We should also get rid of inline qualifier.
+    const tooling::Replacement DeleteFuncBody(
+        Sel.AST.getSourceManager(),
+        CharSourceRange::getTokenRange(
+            *toHalfOpenFileRange(SM, Sel.AST.getASTContext().getLangOpts(),
+                                 Source->getBody()->getSourceRange())),
+        ";");
+    auto HeaderFE = Effect::fileEdit(SM, SM.getMainFileID(),
+                                     tooling::Replacements(DeleteFuncBody));
+    if (!HeaderFE)
+      return HeaderFE.takeError();
+
+    Effect->ApplyEdits.try_emplace(HeaderFE->first,
+                                   std::move(HeaderFE->second));
+    return std::move(*Effect);
   }
 
 private:
diff --git a/clang-tools-extra/clangd/unittests/TweakTesting.cpp b/clang-tools-extra/clangd/unittests/TweakTesting.cpp
--- a/clang-tools-extra/clangd/unittests/TweakTesting.cpp
+++ b/clang-tools-extra/clangd/unittests/TweakTesting.cpp
@@ -127,7 +127,7 @@
         ADD_FAILURE() << "There were changes to additional files, but client "
                          "provided a nullptr for EditedFiles.";
       else
-        EditedFiles->try_emplace(It.first(), Unwrapped.str());
+        EditedFiles->insert_or_assign(It.first(), Unwrapped.str());
     }
   }
   return EditedMainFile;
diff --git a/clang-tools-extra/clangd/unittests/TweakTests.cpp b/clang-tools-extra/clangd/unittests/TweakTests.cpp
--- a/clang-tools-extra/clangd/unittests/TweakTests.cpp
+++ b/clang-tools-extra/clangd/unittests/TweakTests.cpp
@@ -1868,6 +1868,110 @@
     })cpp");
 }
 
+TEST_F(DefineOutlineTest, FailsWithoutSource) {
+  FileName = "Test.hpp";
+  llvm::StringRef Test = "void fo^o() { return; }";
+  llvm::StringRef Expected =
+      "fail: Couldn't find a suitable implementation file.";
+  EXPECT_EQ(apply(Test), Expected);
+}
+
+TEST_F(DefineOutlineTest, ApplyTest) {
+  llvm::StringMap<std::string> EditedFiles;
+  ExtraFiles["Test.cpp"] = "";
+  FileName = "Test.hpp";
+
+  struct {
+    llvm::StringRef Test;
+    llvm::StringRef ExpectedHeader;
+    llvm::StringRef ExpectedSource;
+  } Cases[] = {
+      // Simple check
+      {
+          "void fo^o() { return; }",
+          "void foo() ;",
+          "void foo() { return; }",
+      },
+      // Templated function.
+      {
+          "template <typename T> void fo^o(T, T x) { return; }",
+          "template <typename T> void foo(T, T x) ;",
+          "template <typename T> void foo(T, T x) { return; }",
+      },
+      {
+          "template <typename> void fo^o() { return; }",
+          "template <typename> void foo() ;",
+          "template <typename> void foo() { return; }",
+      },
+      // Template specialization.
+      {
+          R"cpp(
+            template <typename> void foo();
+            template <> void fo^o<int>() { return; })cpp",
+          R"cpp(
+            template <typename> void foo();
+            template <> void foo<int>() ;)cpp",
+          "template <> void foo<int>() { return; }",
+      },
+  };
+  for (const auto &Case : Cases) {
+    SCOPED_TRACE(Case.Test);
+    EXPECT_EQ(apply(Case.Test, &EditedFiles), Case.ExpectedHeader);
+    EXPECT_THAT(EditedFiles, testing::ElementsAre(FileWithContents(
+                                 testPath("Test.cpp"), Case.ExpectedSource)));
+  }
+}
+
+TEST_F(DefineOutlineTest, HandleMacros) {
+  llvm::StringMap<std::string> EditedFiles;
+  ExtraFiles["Test.cpp"] = "";
+  FileName = "Test.hpp";
+
+  struct {
+    llvm::StringRef Test;
+    llvm::StringRef ExpectedHeader;
+    llvm::StringRef ExpectedSource;
+  } Cases[] = {
+      {R"cpp(
+          #define BODY { return; }
+          void f^oo()BODY)cpp",
+       R"cpp(
+          #define BODY { return; }
+          void foo();)cpp",
+       "void foo()BODY"},
+
+      {R"cpp(
+          #define BODY return;
+          void f^oo(){BODY})cpp",
+       R"cpp(
+          #define BODY return;
+          void foo();)cpp",
+       "void foo(){BODY}"},
+
+      {R"cpp(
+          #define TARGET void foo()
+          [[TARGET]]{ return; })cpp",
+       R"cpp(
+          #define TARGET void foo()
+          TARGET;)cpp",
+       "TARGET{ return; }"},
+
+      {R"cpp(
+          #define TARGET foo
+          void [[TARGET]](){ return; })cpp",
+       R"cpp(
+          #define TARGET foo
+          void TARGET();)cpp",
+       "void TARGET(){ return; }"},
+  };
+  for (const auto &Case : Cases) {
+    SCOPED_TRACE(Case.Test);
+    EXPECT_EQ(apply(Case.Test, &EditedFiles), Case.ExpectedHeader);
+    EXPECT_THAT(EditedFiles, testing::ElementsAre(FileWithContents(
+                                 testPath("Test.cpp"), Case.ExpectedSource)));
+  }
+}
+
 } // namespace
 } // namespace clangd
 } // namespace clang