@@ -17,7 +17,7 @@ class Node {}
 /// Represents a method on an [Api].
 class Method extends Node {
   /// Parametric constructor for [Method].
-  Method({this.name, this.returnType, this.argType});
+  Method({this.name, this.returnType, this.argType, this.isAsynchronous});
 
   /// The name of the method.
   String name;
@@ -27,6 +27,9 @@ class Method extends Node {
 
   /// The data-type of the argument.
   String argType;
+
+  /// Whether the receiver of this method is expected to return synchronously or not.
+  bool isAsynchronous;
 }
 
 /// Represents a collection of [Method]s that are hosted ona given [location].
 
@@ -66,9 +66,12 @@ void _writeFlutterApi(Indent indent, Api api,
   indent.write('abstract class ${api.name} ');
   indent.scoped('{', '}', () {
     for (Method func in api.methods) {
+      final bool isAsync = func.isAsynchronous;
+      final String returnType =
+          isAsync ? 'Future<${func.returnType}>' : '${func.returnType}';
       final String argSignature =
           func.argType == 'void' ? '' : '${func.argType} arg';
-      indent.writeln('${func.returnType} ${func.name}($argSignature);');
+      indent.writeln('$returnType ${func.name}($argSignature);');
     }
     indent.write('static void setup(${api.name} api) ');
     indent.scoped('{', '}', () {
@@ -92,6 +95,7 @@ void _writeFlutterApi(Indent indent, Api api,
           indent.scoped('{', '});', () {
             final String argType = func.argType;
             final String returnType = func.returnType;
+            final bool isAsync = func.isAsynchronous;
             String call;
             if (argType == 'void') {
               call = 'api.${func.name}()';
@@ -108,7 +112,11 @@ void _writeFlutterApi(Indent indent, Api api,
                 indent.writeln('return <dynamic, dynamic>{};');
               }
             } else {
-              indent.writeln('final $returnType output = $call;');
+              if (isAsync) {
+                indent.writeln('final $returnType output = await $call;');
+              } else {
+                indent.writeln('final $returnType output = $call;');
+              }
               const String returnExpresion = 'output._toMap()';
               final String returnStatement = isMockHandler
                   ? 'return <dynamic, dynamic>{\'${Keys.result}\': $returnExpresion};'
 
@@ -29,14 +29,30 @@ class JavaOptions {
 
 void _writeHostApi(Indent indent, Api api) {
   assert(api.location == ApiLocation.host);
+
+  if (api.methods.any((Method it) => it.isAsynchronous)) {
+    indent.write('public interface Result<T> ');
+    indent.scoped('{', '}', () {
+      indent.writeln('void success(T result);');
+    });
+    indent.addln('');
+  }
+
   indent.writeln(
       '/** Generated interface from Pigeon that represents a handler of messages from Flutter.*/');
   indent.write('public interface ${api.name} ');
   indent.scoped('{', '}', () {
     for (Method method in api.methods) {
-      final String argSignature =
-          method.argType == 'void' ? '' : '${method.argType} arg';
-      indent.writeln('${method.returnType} ${method.name}($argSignature);');
+      final String returnType =
+          method.isAsynchronous ? 'void' : method.returnType;
+      final List<String> argSignature = <String>[];
+      if (method.argType != 'void') {
+        argSignature.add('${method.argType} arg');
+      }
+      if (method.isAsynchronous) {
+        argSignature.add('Result<${method.returnType}> result');
+      }
+      indent.writeln('$returnType ${method.name}(${argSignature.join(', ')});');
     }
     indent.addln('');
     indent.writeln(
@@ -65,17 +81,26 @@ void _writeHostApi(Indent indent, Api api) {
                   'HashMap<String, HashMap> wrapped = new HashMap<>();');
               indent.write('try ');
               indent.scoped('{', '}', () {
-                String methodArgument;
-                if (argType == 'void') {
-                  methodArgument = '';
-                } else {
+                final List<String> methodArgument = <String>[];
+                if (argType != 'void') {
                   indent.writeln('@SuppressWarnings("ConstantConditions")');
                   indent.writeln(
                       '$argType input = $argType.fromMap((HashMap)message);');
-                  methodArgument = 'input';
+                  methodArgument.add('input');
                 }
-                final String call = 'api.${method.name}($methodArgument)';
-                if (method.returnType == 'void') {
+                if (method.isAsynchronous) {
+                  methodArgument.add(
+                    'result -> { '
+                    'wrapped.put("${Keys.result}", result.toMap()); '
+                    'reply.reply(wrapped); '
+                    '}',
+                  );
+                }
+                final String call =
+                    'api.${method.name}(${methodArgument.join(', ')})';
+                if (method.isAsynchronous) {
+                  indent.writeln('$call;');
+                } else if (method.returnType == 'void') {
                   indent.writeln('$call;');
                   indent.writeln('wrapped.put("${Keys.result}", null);');
                 } else {
@@ -88,8 +113,13 @@ void _writeHostApi(Indent indent, Api api) {
               indent.scoped('{', '}', () {
                 indent.writeln(
                     'wrapped.put("${Keys.error}", wrapError(exception));');
+                if (method.isAsynchronous) {
+                  indent.writeln('reply.reply(wrapped);');
+                }
               });
-              indent.writeln('reply.reply(wrapped);');
+              if (!method.isAsynchronous) {
+                indent.writeln('reply.reply(wrapped);');
+              }
             });
           });
           indent.scoped(null, '}', () {
 
@@ -27,6 +27,13 @@ const List<String> _validTypes = <String>[
   'Map',
 ];
 
+class _Asynchronous {
+  const _Asynchronous();
+}
+
+/// Metadata to annotate a Api method as asynchronous
+const _Asynchronous async = _Asynchronous();
+
 /// Metadata to annotate a Pigeon API implemented by the host-platform.
 ///
 /// The abstract class with this annotation groups a collection of Dart↔host
@@ -224,14 +231,20 @@ class Pigeon {
       final List<Method> functions = <Method>[];
       for (DeclarationMirror declaration in apiMirror.declarations.values) {
         if (declaration is MethodMirror && !declaration.isConstructor) {
+          final bool isAsynchronous =
+              declaration.metadata.any((InstanceMirror it) {
+            return MirrorSystem.getName(it.type.simpleName) ==
+                '${async.runtimeType}';
+          });
           functions.add(Method()
             ..name = MirrorSystem.getName(declaration.simpleName)
             ..argType = declaration.parameters.isEmpty
                 ? 'void'
                 : MirrorSystem.getName(
                     declaration.parameters[0].type.simpleName)
             ..returnType =
-                MirrorSystem.getName(declaration.returnType.simpleName));
+                MirrorSystem.getName(declaration.returnType.simpleName)
+            ..isAsynchronous = isAsynchronous);
         }
       }
       final HostApi hostApi = _getHostApi(apiMirror);
 
@@ -0,0 +1,21 @@
+// Copyright 2020 The Flutter Authors. 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:pigeon/pigeon.dart';
+
+class Value {
+  int number;
+}
+
+@HostApi()
+abstract class Api2Host {
+  @async
+  Value calculate(Value value);
+}
+
+@FlutterApi()
+abstract class Api2Flutter {
+  @async
+  Value calculate(Value value);
+}
@@ -96,6 +96,7 @@ test_pigeon_android ./pigeons/void_arg_host.dart
 test_pigeon_android ./pigeons/void_arg_flutter.dart
 test_pigeon_android ./pigeons/list.dart
 test_pigeon_android ./pigeons/all_datatypes.dart
+test_pigeon_android ./pigeons/async_handlers.dart
 test_pigeon_ios ./pigeons/message.dart
 test_pigeon_ios ./pigeons/host2flutter.dart
 test_pigeon_ios ./pigeons/voidhost.dart
@@ -104,6 +105,8 @@ test_pigeon_ios ./pigeons/void_arg_host.dart
 test_pigeon_ios ./pigeons/void_arg_flutter.dart
 test_pigeon_ios ./pigeons/list.dart
 test_pigeon_ios ./pigeons/all_datatypes.dart
+# Not implemented yet.
+# test_pigeon_ios ./pigeons/async_handlers.dart
 
 ###############################################################################
 # Mock handler flutter tests.