diff --git a/.gitignore b/.gitignore
index 046cfabbc..85cea2750 100644
--- a/.gitignore
+++ b/.gitignore
@@ -7,6 +7,7 @@ pubspec.lock
 # Files generated by dart tools
 .dart_tool
 doc/
+build.lock
 
 # Generated by tool/create_all_test.dart
 tool/test_all.dart
diff --git a/build_runner/CHANGELOG.md b/build_runner/CHANGELOG.md
index d02b45d74..e4d171bdc 100644
--- a/build_runner/CHANGELOG.md
+++ b/build_runner/CHANGELOG.md
@@ -1,8 +1,12 @@
-## 0.7.1+1
+## 0.7.2
 
 - **BUG FIX**: Running the `build_runner` binary without arguments no longer
   causes a crash saying `Could not find an option named "assume-tty".`.
 
+- Builds now automatically generate a `build.lock` file in the package
+  directory while a build is in progress (including watchers and servers), and
+  fail if a lock file is already created.
+
 ## 0.7.1
 
 - Run Builders which write to the source tree before those which write to the
diff --git a/build_runner/bin/build_runner.dart b/build_runner/bin/build_runner.dart
index 41561153d..054327c51 100644
--- a/build_runner/bin/build_runner.dart
+++ b/build_runner/bin/build_runner.dart
@@ -9,11 +9,18 @@ import 'package:io/io.dart';
 import 'package:logging/logging.dart';
 
 import 'package:build_runner/src/build_script_generate/build_script_generate.dart';
+import 'package:build_runner/src/generate/exceptions.dart';
 import 'package:build_runner/src/logging/std_io_logging.dart';
 
 Future<Null> main(List<String> args) async {
   var logListener = Logger.root.onRecord.listen(stdIOLogListener);
-  await ensureBuildScript();
+  try {
+    await ensureBuildScript();
+  } on ConcurrentBuildException catch (e) {
+    Logger.root.log(Level.SEVERE, '$e');
+    exitCode = 1;
+    return;
+  }
   var dart = Platform.resolvedExecutable;
   final innerArgs = [scriptLocation]..addAll(args);
   if (stdioType(stdin) == StdioType.TERMINAL &&
diff --git a/build_runner/lib/build_runner.dart b/build_runner/lib/build_runner.dart
index 138975a0a..273fb00b8 100644
--- a/build_runner/lib/build_runner.dart
+++ b/build_runner/lib/build_runner.dart
@@ -7,6 +7,7 @@ export 'src/asset/writer.dart';
 export 'src/entrypoint/run.dart' show run;
 export 'src/generate/build.dart';
 export 'src/generate/build_result.dart';
+export 'src/generate/lock_file.dart' show clearLock, createLock;
 export 'src/generate/performance_tracker.dart'
     show BuildPerformance, BuildActionPerformance;
 export 'src/package_graph/apply_builders.dart'
diff --git a/build_runner/lib/src/build_script_generate/build_script_generate.dart b/build_runner/lib/src/build_script_generate/build_script_generate.dart
index d3be71592..6a590a349 100644
--- a/build_runner/lib/src/build_script_generate/build_script_generate.dart
+++ b/build_runner/lib/src/build_script_generate/build_script_generate.dart
@@ -38,11 +38,12 @@ Future<String> _generateBuildScript() async {
 /// Finds expressions to create all the [BuilderApplication] instances that
 /// should be applied packages in the build.
 ///
-/// Adds `apply` expressions based on the BuildefDefinitions from any package
+/// Adds `apply` expressions based on the BuilderDefinitions from any package
 /// which has a `build.yaml`.
 Future<Iterable<Expression>> _findBuilderApplications() async {
   final builderApplications = <Expression>[];
   final packageGraph = new PackageGraph.forThisPackage();
+  await createLock(packageGraph);
   final orderedPackages = stronglyConnectedComponents<String, PackageNode>(
           [packageGraph.root], (node) => node.name, (node) => node.dependencies)
       .expand((c) => c);
diff --git a/build_runner/lib/src/generate/build_impl.dart b/build_runner/lib/src/generate/build_impl.dart
index 2dc0deeb9..56da94673 100644
--- a/build_runner/lib/src/generate/build_impl.dart
+++ b/build_runner/lib/src/generate/build_impl.dart
@@ -30,6 +30,7 @@ import 'build_result.dart';
 import 'exceptions.dart';
 import 'fold_frames.dart';
 import 'heartbeat.dart';
+import 'lock_file.dart';
 import 'options.dart';
 import 'performance_tracker.dart';
 import 'phase.dart';
@@ -71,6 +72,7 @@ Future<BuildResult> build(
 
   await terminator.cancel();
   await options.logListener.cancel();
+  await clearLock(options.packageGraph);
   return result;
 }
 
diff --git a/build_runner/lib/src/generate/exceptions.dart b/build_runner/lib/src/generate/exceptions.dart
index 910b1779c..367b29692 100644
--- a/build_runner/lib/src/generate/exceptions.dart
+++ b/build_runner/lib/src/generate/exceptions.dart
@@ -8,11 +8,14 @@ import 'package:build_runner/src/generate/phase.dart';
 import 'phase.dart';
 
 class ConcurrentBuildException implements Exception {
-  const ConcurrentBuildException();
+  final String lockFilePath;
+
+  const ConcurrentBuildException(this.lockFilePath);
 
   @override
-  String toString() =>
-      'ConcurrentBuildException: Only one build may be running at a time.';
+  String toString() => 'Only one build may be running at a time!\n'
+      'If a build process has failed, or you want to remove the lock \n'
+      'manually simple delete the file "$lockFilePath".';
 }
 
 abstract class FatalBuildException implements Exception {
diff --git a/build_runner/lib/src/generate/lock_file.dart b/build_runner/lib/src/generate/lock_file.dart
new file mode 100644
index 000000000..3cd7f1107
--- /dev/null
+++ b/build_runner/lib/src/generate/lock_file.dart
@@ -0,0 +1,58 @@
+// Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'dart:async';
+import 'dart:io';
+
+import 'package:crypto/crypto.dart';
+import 'package:path/path.dart' as p;
+
+import '../generate/exceptions.dart';
+import '../package_graph/package_graph.dart';
+
+/// Returns a simple string hash used to determine if a lock file is owned.
+String _createHash() => md5
+    .convert(pid.toString().codeUnits)
+    .bytes
+    .map((b) => b.toRadixString(16))
+    .join('');
+
+/// Returns a future that completes with a hash when [graph] could be locked.
+///
+/// This same hash is expected to be used with [clearLock] when the build
+/// process is complete. Throws a `ConcurrentBuildException` if the lock could
+/// not be created.
+Future<String> createLock(PackageGraph graph, {String hash}) async {
+  // For package graphs without a root (usually for testing), just succeed.
+  hash ??= _createHash();
+  if (graph?.root?.path == null) {
+    return hash;
+  }
+  final path = p.join(graph.root.path, 'build.lock');
+  final file = new File(path);
+  if (await file.exists()) {
+    throw new ConcurrentBuildException(file.absolute.path);
+  }
+  await file.writeAsString(hash);
+  return hash;
+}
+
+/// Returns a future that completes after clearing the lock file for [graph].
+Future<Null> clearLock(PackageGraph graph, {String hash}) async {
+  // For package graphs without a root (usually for testing), just succeed.
+  if (graph?.root?.path == null) {
+    return;
+  }
+  final path = p.join(graph.root.path, 'build.lock');
+  final file = new File(path);
+  if (!await file.exists()) {
+    return;
+  }
+  final input = await file.readAsString();
+  hash ??= _createHash();
+  if (hash != input) {
+    throw new ConcurrentBuildException(file.absolute.path);
+  }
+  await file.delete();
+}
diff --git a/build_runner/lib/src/generate/watch_impl.dart b/build_runner/lib/src/generate/watch_impl.dart
index 0743a2c2e..a47f1f5a8 100644
--- a/build_runner/lib/src/generate/watch_impl.dart
+++ b/build_runner/lib/src/generate/watch_impl.dart
@@ -25,6 +25,7 @@ import 'build_definition.dart';
 import 'build_impl.dart';
 import 'build_result.dart';
 import 'directory_watcher_factory.dart';
+import 'lock_file.dart';
 import 'options.dart';
 import 'phase.dart';
 import 'terminator.dart';
@@ -65,12 +66,14 @@ Future<ServeHandler> watch(
   final buildActions = await createBuildActions(options.packageGraph, builders,
       overrideBuildConfig: overrideBuildConfig);
 
-  var watch = runWatch(options, buildActions, terminator.shouldTerminate);
+  var watch = runWatch(options, buildActions,
+      terminator.shouldTerminate.then((_) => clearLock(options.packageGraph)));
 
   // ignore: unawaited_futures
   watch.buildResults.drain().then((_) async {
     await terminator.cancel();
     await options.logListener.cancel();
+    await clearLock(options.packageGraph);
   });
 
   return createServeHandler(watch);
@@ -149,6 +152,7 @@ class WatchImpl implements BuildState {
             .hasBeenUpdated(mergedChanges.keys.toSet())) {
           fatalBuildCompleter.complete();
           _logger.severe('Terminating builds due to build script update');
+          await clearLock(options.packageGraph);
           return new BuildResult(BuildStatus.failure, []);
         }
       }
@@ -182,6 +186,7 @@ class WatchImpl implements BuildState {
           await currentBuild;
           await _buildDefinition.resourceManager.beforeExit();
           await controller.close();
+          await clearLock(options.packageGraph);
           _logger.info('Builds finished. Safe to exit\n');
         });
 
diff --git a/build_runner/pubspec.yaml b/build_runner/pubspec.yaml
index e9123abbd..c41ad4a39 100644
--- a/build_runner/pubspec.yaml
+++ b/build_runner/pubspec.yaml
@@ -1,5 +1,5 @@
 name: build_runner
-version: 0.7.1+1
+version: 0.7.2
 description: Tools to write binaries that run builders.
 author: Dart Team <misc@dartlang.org>
 homepage: https://github.com/dart-lang/build
diff --git a/build_runner/test/generate/lock_file_test.dart b/build_runner/test/generate/lock_file_test.dart
new file mode 100644
index 000000000..e4eb2d7f2
--- /dev/null
+++ b/build_runner/test/generate/lock_file_test.dart
@@ -0,0 +1,78 @@
+// Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'package:build_runner/src/generate/exceptions.dart';
+import 'package:build_runner/src/generate/lock_file.dart';
+import 'package:build_runner/src/package_graph/package_graph.dart';
+import 'package:path/path.dart' as p;
+import 'package:test/test.dart';
+import 'package:test_descriptor/test_descriptor.dart' as d;
+
+import '../common/descriptors.dart';
+
+void main() {
+  PackageGraph graph;
+
+  setUp(() async {
+    await d.dir('pkg', [
+      d.dir('lib'),
+      d.file('.packages', 'pkg:./lib/'),
+      await pubspec('pkg'),
+    ]).create();
+
+    final root = new PackageNode('pkg', p.join(d.sandbox, 'pkg'), isRoot: true);
+    graph = new PackageGraph.fromRoot(root);
+    expect(graph.root.path, isNotNull);
+  });
+
+  test('should create a lock file', () async {
+    // Start with no lock file.
+    await d.dir('pkg', [
+      d.nothing('build.lock'),
+    ]).validate();
+
+    // Create a lock file.
+    await createLock(graph, hash: '1234');
+
+    // Expect a lock file.
+    await d.dir('pkg', [
+      d.file('build.lock', '1234'),
+    ]).validate();
+  });
+
+  test('should throw if a lock file already exists', () async {
+    await d.dir('pkg', [
+      d.file('build.lock', '1234'),
+    ]).create();
+
+    expect(createLock(graph), throwsConcurrentBuildException);
+  });
+
+  test('should remove a lock file', () async {
+    await d.dir('pkg', [
+      d.file('build.lock', '1234'),
+    ]).create();
+
+    await clearLock(graph, hash: '1234');
+
+    await d.dir('pkg', [
+      d.nothing('build.lock'),
+    ]).validate();
+  });
+
+  test('should throw on removing a lock file with a wrong hash', () async {
+    await d.dir('pkg', [
+      d.file('build.lock', '1234'),
+    ]).create();
+
+    expect(clearLock(graph, hash: 'ABCD'), throwsConcurrentBuildException);
+
+    await d.dir('pkg', [
+      d.file('build.lock', '1234'),
+    ]).validate();
+  });
+}
+
+final throwsConcurrentBuildException =
+    throwsA(const isInstanceOf<ConcurrentBuildException>());