diff --git a/packages/flutter/lib/rendering.dart b/packages/flutter/lib/rendering.dart
index 9e93e161c4..106881a15d 100644
--- a/packages/flutter/lib/rendering.dart
+++ b/packages/flutter/lib/rendering.dart
@@ -22,6 +22,7 @@
 /// initialized with those features.
 library rendering;
 
+export 'src/rendering/animated_size.dart';
 export 'src/rendering/auto_layout.dart';
 export 'src/rendering/binding.dart';
 export 'src/rendering/block.dart';
diff --git a/packages/flutter/lib/src/rendering/animated_size.dart b/packages/flutter/lib/src/rendering/animated_size.dart
new file mode 100644
index 0000000000..d050f87d28
--- /dev/null
+++ b/packages/flutter/lib/src/rendering/animated_size.dart
@@ -0,0 +1,139 @@
+// Copyright 2016 The Chromium 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:flutter/animation.dart';
+import 'package:meta/meta.dart';
+
+import 'box.dart';
+import 'object.dart';
+import 'shifted_box.dart';
+
+/// A render object that animates its size to its child's size over a given
+/// [duration] and with a given [curve]. In case the child's size is animating
+/// as opposed to abruptly changing size, the parent behaves like a normal
+/// container.
+///
+/// In case the child overflows the current animated size of the parent, it gets
+/// clipped automatically.
+class RenderAnimatedSize extends RenderAligningShiftedBox {
+  /// Creates a render object that animates its size to match its child.
+  /// The [duration] and [curve] arguments define the animation. The [alignment]
+  /// argument is used to align the child in the case where the parent is not
+  /// (yet) the same size as the child.
+  ///
+  /// The arguments [duration], [curve], and [alignment] should not be null.
+  RenderAnimatedSize({
+    Curve curve: Curves.linear,
+    RenderBox child,
+    FractionalOffset alignment: FractionalOffset.center,
+    @required Duration duration
+  }) : super(child: child, alignment: alignment) {
+    assert(duration != null);
+    assert(curve != null);
+    _controller = new AnimationController(
+      duration: duration
+    )..addListener(() {
+      if (_controller.value != _lastValue)
+        markNeedsLayout();
+    });
+    _animation = new CurvedAnimation(
+      parent: _controller,
+      curve: curve
+    );
+  }
+
+  AnimationController _controller;
+  CurvedAnimation _animation;
+  SizeTween _sizeTween = new SizeTween();
+  bool _didChangeTargetSizeLastFrame = false;
+  bool _hasVisualOverflow;
+  double _lastValue;
+
+  /// The duration of the animation.
+  Duration get duration => _controller.duration;
+  set duration(Duration value) {
+    assert(value != null);
+    if (value == _controller.duration)
+      return;
+    _controller.duration = value;
+  }
+
+  /// The curve of the animation.
+  Curve get curve => _animation.curve;
+  set curve(Curve value) {
+    assert(value != null);
+    if (value == _animation.curve)
+      return;
+    _animation.curve = value;
+  }
+
+  @override
+  void attach(PipelineOwner owner) {
+    super.attach(owner);
+    if (_animatedSize != _sizeTween.end && !_controller.isAnimating)
+      _controller.forward();
+  }
+
+  @override
+  void detach() {
+    _controller.stop();
+    super.detach();
+  }
+
+  Size get _animatedSize {
+    return _sizeTween.evaluate(_animation);
+  }
+
+  @override
+  void performLayout() {
+    _lastValue = _controller.value;
+    _hasVisualOverflow = false;
+
+    if (child == null) {
+      size = _sizeTween.begin = _sizeTween.end = constraints.smallest;
+      return;
+    }
+
+    child.layout(constraints, parentUsesSize: true);
+    if (_sizeTween.end != child.size) {
+      _sizeTween.begin = _animatedSize ?? child.size;
+      _sizeTween.end = child.size;
+
+      if (_didChangeTargetSizeLastFrame) {
+        size = child.size;
+        _controller.stop();
+      } else {
+        // Don't register first change (i.e. when _targetSize == _sourceSize)
+        // as a last-frame change.
+        if (_sizeTween.end != _sizeTween.begin)
+          _didChangeTargetSizeLastFrame = true;
+
+        _lastValue = 0.0;
+        _controller.forward(from: 0.0);
+
+        size = constraints.constrain(_animatedSize);
+      }
+    } else {
+      _didChangeTargetSizeLastFrame = false;
+
+      size = constraints.constrain(_animatedSize);
+    }
+
+    alignChild();
+
+    if (size.width < _sizeTween.end.width ||
+        size.height < _sizeTween.end.height)
+      _hasVisualOverflow = true;
+  }
+
+  @override
+  void paint(PaintingContext context, Offset offset) {
+    if (child != null && _hasVisualOverflow) {
+      final Rect rect = Point.origin & size;
+      context.pushClipRect(needsCompositing, offset, rect, super.paint);
+    } else {
+      super.paint(context, offset);
+    }
+  }
+}
diff --git a/packages/flutter/lib/src/widgets/animated_size.dart b/packages/flutter/lib/src/widgets/animated_size.dart
new file mode 100644
index 0000000000..96af3857b6
--- /dev/null
+++ b/packages/flutter/lib/src/widgets/animated_size.dart
@@ -0,0 +1,62 @@
+// Copyright 2016 The Chromium 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:flutter/rendering.dart';
+import 'package:meta/meta.dart';
+
+import 'basic.dart';
+import 'framework.dart';
+
+/// Animated widget that automatically transitions its size over a given
+/// duration whenever the given child's size changes.
+class AnimatedSize extends SingleChildRenderObjectWidget {
+  /// Creates a widget that animates its size to match that of its child.
+  ///
+  /// The [curve] and [duration] arguments must not be null.
+  AnimatedSize({
+    Key key,
+    Widget child,
+    this.alignment: FractionalOffset.center,
+    this.curve: Curves.linear,
+    @required this.duration
+  }) : super(key: key, child: child);
+
+  /// The alignment of the child within the parent when the parent is not yet
+  /// the same size as the child.
+  ///
+  /// The x and y values of the alignment control the horizontal and vertical
+  /// alignment, respectively. An x value of 0.0 means that the left edge of
+  /// the child is aligned with the left edge of the parent whereas an x value
+  /// of 1.0 means that the right edge of the child is aligned with the right
+  /// edge of the parent. Other values interpolate (and extrapolate) linearly.
+  /// For example, a value of 0.5 means that the center of the child is aligned
+  /// with the center of the parent.
+  final FractionalOffset alignment;
+
+  /// The animation curve when transitioning this widget's size to match the
+  /// child's size.
+  final Curve curve;
+
+  /// The duration when transitioning this widget's size to match the child's
+  /// size.
+  final Duration duration;
+
+  @override
+  RenderAnimatedSize createRenderObject(BuildContext context) {
+    return new RenderAnimatedSize(
+      alignment: alignment,
+      duration: duration,
+      curve: curve
+    );
+  }
+
+  @override
+  void updateRenderObject(BuildContext context,
+                          RenderAnimatedSize renderObject) {
+    renderObject
+      ..alignment = alignment
+      ..duration = duration
+      ..curve = curve;
+  }
+}
diff --git a/packages/flutter/lib/src/widgets/implicit_animations.dart b/packages/flutter/lib/src/widgets/implicit_animations.dart
index 0c4e3be3c1..b2d16c2d2c 100644
--- a/packages/flutter/lib/src/widgets/implicit_animations.dart
+++ b/packages/flutter/lib/src/widgets/implicit_animations.dart
@@ -2,13 +2,13 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'package:meta/meta.dart';
+import 'package:vector_math/vector_math_64.dart';
+
 import 'basic.dart';
 import 'container.dart';
 import 'framework.dart';
 
-import 'package:meta/meta.dart';
-import 'package:vector_math/vector_math_64.dart';
-
 /// An interpolation between two [BoxConstraint]s.
 class BoxConstraintsTween extends Tween<BoxConstraints> {
   /// Creates a box constraints tween.
diff --git a/packages/flutter/lib/widgets.dart b/packages/flutter/lib/widgets.dart
index f944f3e6c1..684159137c 100644
--- a/packages/flutter/lib/widgets.dart
+++ b/packages/flutter/lib/widgets.dart
@@ -8,6 +8,7 @@
 /// To use, import `package:flutter/widgets.dart`.
 library widgets;
 
+export 'src/widgets/animated_size.dart';
 export 'src/widgets/app.dart';
 export 'src/widgets/auto_layout.dart';
 export 'src/widgets/banner.dart';
diff --git a/packages/flutter/test/widget/animated_size_test.dart b/packages/flutter/test/widget/animated_size_test.dart
new file mode 100644
index 0000000000..aabc499981
--- /dev/null
+++ b/packages/flutter/test/widget/animated_size_test.dart
@@ -0,0 +1,172 @@
+// Copyright 2016 The Chromium 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:flutter_test/flutter_test.dart';
+import 'package:flutter/rendering.dart';
+import 'package:flutter/widgets.dart';
+
+class TestPaintingContext implements PaintingContext {
+  final List<Invocation> invocations = <Invocation>[];
+
+  @override
+    void noSuchMethod(Invocation invocation) {
+      invocations.add(invocation);
+    }
+}
+
+void main() {
+  testWidgets('AnimatedSize test', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      new Center(
+        child: new AnimatedSize(
+          duration: const Duration(milliseconds: 200),
+          child: new SizedBox(
+            width: 100.0,
+            height: 100.0
+          )
+        )
+      )
+    );
+
+    RenderBox box = tester.renderObject(find.byType(AnimatedSize));
+    expect(box.size.width, equals(100.0));
+    expect(box.size.height, equals(100.0));
+
+    await tester.pumpWidget(
+      new Center(
+        child: new AnimatedSize(
+          duration: new Duration(milliseconds: 200),
+          child: new SizedBox(
+            width: 200.0,
+            height: 200.0
+          )
+        )
+      )
+    );
+
+    await tester.pump(const Duration(milliseconds: 100));
+    box = tester.renderObject(find.byType(AnimatedSize));
+    expect(box.size.width, equals(150.0));
+    expect(box.size.height, equals(150.0));
+
+    TestPaintingContext context = new TestPaintingContext();
+    box.paint(context, Offset.zero);
+    expect(context.invocations.first.memberName, equals(#pushClipRect));
+
+    await tester.pump(const Duration(milliseconds: 100));
+    box = tester.renderObject(find.byType(AnimatedSize));
+    expect(box.size.width, equals(200.0));
+    expect(box.size.height, equals(200.0));
+
+    await tester.pumpWidget(
+      new Center(
+        child: new AnimatedSize(
+          duration: new Duration(milliseconds: 200),
+          child: new SizedBox(
+            width: 100.0,
+            height: 100.0
+          )
+        )
+      )
+    );
+
+    await tester.pump(const Duration(milliseconds: 100));
+    box = tester.renderObject(find.byType(AnimatedSize));
+    expect(box.size.width, equals(150.0));
+    expect(box.size.height, equals(150.0));
+
+    context = new TestPaintingContext();
+    box.paint(context, Offset.zero);
+    expect(context.invocations.first.memberName, equals(#paintChild));
+
+    await tester.pump(const Duration(milliseconds: 100));
+    box = tester.renderObject(find.byType(AnimatedSize));
+    expect(box.size.width, equals(100.0));
+    expect(box.size.height, equals(100.0));
+  });
+
+  testWidgets('AnimatedSize constrained test', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      new Center(
+        child: new SizedBox (
+          width: 100.0,
+          height: 100.0,
+          child: new AnimatedSize(
+            duration: const Duration(milliseconds: 200),
+            child: new SizedBox(
+              width: 100.0,
+              height: 100.0
+            )
+          )
+        )
+      )
+    );
+
+    RenderBox box = tester.renderObject(find.byType(AnimatedSize));
+    expect(box.size.width, equals(100.0));
+    expect(box.size.height, equals(100.0));
+
+    await tester.pumpWidget(
+      new Center(
+        child: new SizedBox (
+          width: 100.0,
+          height: 100.0,
+          child: new AnimatedSize(
+            duration: const Duration(milliseconds: 200),
+            child: new SizedBox(
+              width: 200.0,
+              height: 200.0
+            )
+          )
+        )
+      )
+    );
+
+    await tester.pump(const Duration(milliseconds: 100));
+    box = tester.renderObject(find.byType(AnimatedSize));
+    expect(box.size.width, equals(100.0));
+    expect(box.size.height, equals(100.0));
+  });
+
+  testWidgets('AnimatedSize with AnimatedContainer', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      new Center(
+        child: new AnimatedSize(
+          duration: const Duration(milliseconds: 200),
+          child: new AnimatedContainer(
+            duration: const Duration(milliseconds: 100),
+            width: 100.0,
+            height: 100.0
+          )
+        )
+      )
+    );
+
+    RenderBox box = tester.renderObject(find.byType(AnimatedSize));
+    expect(box.size.width, equals(100.0));
+    expect(box.size.height, equals(100.0));
+
+    await tester.pumpWidget(
+      new Center(
+        child: new AnimatedSize(
+          duration: const Duration(milliseconds: 200),
+          child: new AnimatedContainer(
+            duration: const Duration(milliseconds: 100),
+            width: 200.0,
+            height: 200.0
+          )
+        )
+      )
+    );
+
+    await tester.pump(const Duration(milliseconds: 1)); // register change
+    await tester.pump(const Duration(milliseconds: 49));
+    expect(box.size.width, equals(150.0));
+    expect(box.size.height, equals(150.0));
+    await tester.pump(const Duration(milliseconds: 50));
+    box = tester.renderObject(find.byType(AnimatedSize));
+    expect(box.size.width, equals(200.0));
+    expect(box.size.height, equals(200.0));
+  });
+}