diff --git a/examples/widgets/smooth_resize.dart b/examples/widgets/smooth_resize.dart
new file mode 100644
index 0000000000..d493476a92
--- /dev/null
+++ b/examples/widgets/smooth_resize.dart
@@ -0,0 +1,115 @@
+// Copyright 2015 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:flutter/material.dart';
+import 'package:flutter/rendering.dart';
+
+final List<Map<int, Color>> _kColors = [
+  Colors.amber,
+  Colors.yellow,
+  Colors.blue,
+  Colors.purple,
+  Colors.indigo,
+  Colors.deepOrange,
+];
+
+class SmoothBlock extends StatefulComponent {
+  SmoothBlock({ this.color });
+
+  final Map<int, Color> color;
+
+  SmoothBlockState createState() => new SmoothBlockState();
+}
+
+class CardTransition extends StatelessComponent {
+  CardTransition({
+    this.child,
+    this.performance,
+    this.x,
+    this.opacity,
+    this.scale
+  });
+
+  final Widget child;
+  final Performance performance;
+  final AnimatedValue<double> x;
+  final AnimatedValue<double> opacity;
+  final AnimatedValue<double> scale;
+
+  Widget build(BuildContext context) {
+
+    return new BuilderTransition(
+      performance: performance,
+      variables: [x, opacity, scale],
+      builder: (BuildContext context) {
+        Matrix4 transform = new Matrix4.identity()
+          ..translate(x.value)
+          ..scale(scale.value, scale.value);
+        return new Opacity(
+          opacity: opacity.value,
+          child: new Transform(
+            transform: transform,
+            child: child
+          )
+        );
+      }
+    );
+  }
+}
+
+class SmoothBlockState extends State<SmoothBlock> {
+
+  double _height = 100.0;
+
+  Widget _handleEnter(PerformanceView performance, Widget child) {
+    return new CardTransition(
+      x: new AnimatedValue<double>(-200.0, end: 0.0, curve: Curves.ease),
+      opacity: new AnimatedValue<double>(0.0, end: 1.0, curve: Curves.ease),
+      scale: new AnimatedValue<double>(0.8, end: 1.0, curve: Curves.ease),
+      performance: performance,
+      child: child
+    );
+  }
+
+  Widget _handleExit(PerformanceView performance, Widget child) {
+    return new CardTransition(
+      x: new AnimatedValue<double>(0.0, end: 200.0, curve: Curves.ease),
+      opacity: new AnimatedValue<double>(1.0, end: 0.0, curve: Curves.ease),
+      scale: new AnimatedValue<double>(1.0, end: 0.8, curve: Curves.ease),
+      performance: performance,
+      child: child
+    );
+  }
+
+  Widget build(BuildContext context) {
+    return new GestureDetector(
+      onTap: () {
+        setState(() {
+          _height = _height == 100.0 ? 200.0 : 100.0;
+        });
+      },
+      child: new EnterExitTransition(
+        duration: const Duration(milliseconds: 1500),
+        onEnter: _handleEnter,
+        onExit: _handleExit,
+        child: new Container(
+          key: new ValueKey(_height),
+          height: _height,
+          decoration: new BoxDecoration(backgroundColor: config.color[_height.floor() * 4])
+        )
+      )
+    );
+  }
+}
+
+class SmoothResizeDemo extends StatelessComponent {
+  Widget build(BuildContext context) {
+    return new Block(_kColors.map((Map<int, Color> color) => new SmoothBlock(color: color)).toList());
+  }
+}
+
+void main() {
+  runApp(new SmoothResizeDemo());
+}
diff --git a/packages/flutter/lib/src/rendering/overflow.dart b/packages/flutter/lib/src/rendering/overflow.dart
index 586cdeeba7..24b3ec081b 100644
--- a/packages/flutter/lib/src/rendering/overflow.dart
+++ b/packages/flutter/lib/src/rendering/overflow.dart
@@ -146,6 +146,67 @@ class RenderOverflowBox extends RenderBox with RenderObjectWithChildMixin<Render
   }
 }
 
+/// A render box that's a specific size but passes its original constraints through to its child, which will probably overflow
+class RenderSizedOverflowBox extends RenderBox with RenderObjectWithChildMixin<RenderBox> {
+  RenderSizedOverflowBox({
+    RenderBox child,
+    Size requestedSize
+  }) : _requestedSize = requestedSize {
+    assert(requestedSize != null);
+    this.child = child;
+  }
+
+  /// The size this render box should attempt to be.
+  Size get requestedSize => _requestedSize;
+  Size _requestedSize;
+  void set requestedSize (Size value) {
+    assert(value != null);
+    if (_requestedSize == value)
+      return;
+    _requestedSize = value;
+    markNeedsLayout();
+  }
+
+  double getMinIntrinsicWidth(BoxConstraints constraints) {
+    return constraints.constrainWidth(_requestedSize.width);
+  }
+
+  double getMaxIntrinsicWidth(BoxConstraints constraints) {
+    return constraints.constrainWidth(_requestedSize.width);
+  }
+
+  double getMinIntrinsicHeight(BoxConstraints constraints) {
+    return constraints.constrainWidth(_requestedSize.height);
+  }
+
+  double getMaxIntrinsicHeight(BoxConstraints constraints) {
+    return constraints.constrainWidth(_requestedSize.height);
+  }
+
+  double computeDistanceToActualBaseline(TextBaseline baseline) {
+    if (child != null)
+      return child.getDistanceToActualBaseline(baseline);
+    return super.computeDistanceToActualBaseline(baseline);
+  }
+
+  void performLayout() {
+    size = constraints.constrain(_requestedSize);
+    if (child != null)
+      child.layout(constraints);
+  }
+
+  void hitTestChildren(HitTestResult result, { Point position }) {
+    if (child != null)
+      child.hitTest(result, position: position);
+    else
+      super.hitTestChildren(result, position: position);
+  }
+
+  void paint(PaintingContext context, Offset offset) {
+    if (child != null)
+      context.paintChild(child, offset.toPoint());
+  }
+}
 
 /// Lays the child out as if it was in the tree, but without painting anything,
 /// without making the child available for hit testing, and without taking any
diff --git a/packages/flutter/lib/src/rendering/proxy_box.dart b/packages/flutter/lib/src/rendering/proxy_box.dart
index b98ba838a0..1b50c2b4a9 100644
--- a/packages/flutter/lib/src/rendering/proxy_box.dart
+++ b/packages/flutter/lib/src/rendering/proxy_box.dart
@@ -861,8 +861,12 @@ class RenderSizeObserver extends RenderProxyBox {
   void performLayout() {
     Size oldSize = hasSize ? size : null;
     super.performLayout();
-    if (oldSize != size)
-      onSizeChanged(size);
+    if (oldSize != size) {
+      // We make a copy of the Size object here because if we leak a _DebugSize
+      // object out of the render tree, we can get confused later if it comes
+      // back and gets set as the size property of a RenderBox.
+      onSizeChanged(new Size(size.width, size.height));
+    }
   }
 }
 
diff --git a/packages/flutter/lib/src/widgets/basic.dart b/packages/flutter/lib/src/widgets/basic.dart
index 8e21acc985..c9ed90652a 100644
--- a/packages/flutter/lib/src/widgets/basic.dart
+++ b/packages/flutter/lib/src/widgets/basic.dart
@@ -346,6 +346,19 @@ class OverflowBox extends OneChildRenderObjectWidget {
   }
 }
 
+class SizedOverflowBox extends OneChildRenderObjectWidget {
+  SizedOverflowBox({ Key key, this.size, Widget child })
+    : super(key: key, child: child);
+
+  final Size size;
+
+  RenderSizedOverflowBox createRenderObject() => new RenderSizedOverflowBox(requestedSize: size);
+
+  void updateRenderObject(RenderSizedOverflowBox renderObject, SizedOverflowBox oldWidget) {
+    renderObject.requestedSize = size;
+  }
+}
+
 class OffStage extends OneChildRenderObjectWidget {
   OffStage({ Key key, Widget child })
     : super(key: key, child: child);
diff --git a/packages/flutter/lib/src/widgets/enter_exit_transition.dart b/packages/flutter/lib/src/widgets/enter_exit_transition.dart
new file mode 100644
index 0000000000..7b6dbee953
--- /dev/null
+++ b/packages/flutter/lib/src/widgets/enter_exit_transition.dart
@@ -0,0 +1,185 @@
+// Copyright 2015 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 'dart:async';
+
+import 'package:flutter/animation.dart';
+
+import 'basic.dart';
+import 'framework.dart';
+
+class SmoothlyResizingOverflowBox extends StatefulComponent {
+  SmoothlyResizingOverflowBox({
+    Key key,
+    this.child,
+    this.size,
+    this.duration,
+    this.curve: Curves.linear
+  }) : super(key: key) {
+    assert(duration != null);
+    assert(curve != null);
+  }
+
+  final Widget child;
+  final Size size;
+  final Duration duration;
+  final Curve curve;
+
+  _SmoothlyResizingOverflowBoxState createState() => new _SmoothlyResizingOverflowBoxState();
+}
+
+class _SmoothlyResizingOverflowBoxState extends State<SmoothlyResizingOverflowBox> {
+  ValuePerformance<Size> _size;
+
+  void initState() {
+    super.initState();
+    _size = new ValuePerformance(
+      variable: new AnimatedSizeValue(config.size, curve: config.curve),
+      duration: config.duration
+    )..addListener(() {
+      setState(() {});
+    });
+  }
+
+  void didUpdateConfig(SmoothlyResizingOverflowBox oldConfig) {
+    _size.duration = config.duration;
+    _size.variable.curve = config.curve;
+    if (config.size != oldConfig.size) {
+      AnimatedSizeValue variable = _size.variable;
+      variable.begin = variable.value;
+      variable.end = config.size;
+      _size.progress = 0.0;
+      _size.play();
+    }
+  }
+
+  void dispose() {
+    _size.stop();
+    super.dispose();
+  }
+
+  Widget build(BuildContext context) {
+    return new SizedOverflowBox(
+      size: _size.value,
+      child: config.child
+    );
+  }
+}
+
+class _Entry {
+  _Entry({
+    this.child,
+    this.enterPerformance,
+    this.enterTransition
+  });
+
+  final Widget child;
+  final Performance enterPerformance;
+  final Widget enterTransition;
+
+  Size childSize = Size.zero;
+
+  Performance exitPerformance;
+  Widget exitTransition;
+
+  Widget get currentTransition => exitTransition ?? enterTransition;
+
+  void dispose() {
+    enterPerformance?.stop();
+    exitPerformance?.stop();
+  }
+}
+
+typedef Widget TransitionBuilderCallback(PerformanceView performance, Widget child);
+
+class EnterExitTransition extends StatefulComponent {
+  EnterExitTransition({
+    Key key,
+    this.child,
+    this.duration,
+    this.curve: Curves.linear,
+    this.onEnter,
+    this.onExit
+  }) : super(key: key) {
+    assert(child != null);
+    assert(duration != null);
+    assert(curve != null);
+    assert(onEnter != null);
+    assert(onExit != null);
+  }
+
+  final Widget child;
+  final Duration duration;
+  final Curve curve;
+  final TransitionBuilderCallback onEnter;
+  final TransitionBuilderCallback onExit;
+
+  _EnterExitTransitionState createState() => new _EnterExitTransitionState();
+}
+
+class _EnterExitTransitionState extends State<EnterExitTransition> {
+  final List<_Entry> _entries = new List<_Entry>();
+
+  void initState() {
+    super.initState();
+    _entries.add(_createEnterTransition());
+  }
+
+  _Entry _createEnterTransition() {
+    Performance enterPerformance = new Performance(duration: config.duration)..play();
+    return new _Entry(
+      child: config.child,
+      enterPerformance: enterPerformance,
+      enterTransition: config.onEnter(enterPerformance, new KeyedSubtree(
+        key: new GlobalKey(),
+        child: config.child
+      ))
+    );
+  }
+
+  Future _createExitTransition(_Entry entry) async {
+    Performance exitPerformance = new Performance(duration: config.duration);
+    entry
+      ..exitPerformance = exitPerformance
+      ..exitTransition = config.onExit(exitPerformance, entry.enterTransition);
+    await exitPerformance.play();
+    if (!mounted)
+      return;
+    setState(() {
+      _entries.remove(entry);
+    });
+  }
+
+  void didUpdateConfig(EnterExitTransition oldConfig) {
+    if (config.child.key != oldConfig.child.key) {
+      _createExitTransition(_entries.last);
+      _entries.add(_createEnterTransition());
+    }
+  }
+
+  void dispose() {
+    for (_Entry entry in new List<_Entry>.from(_entries))
+      entry.dispose();
+    super.dispose();
+  }
+
+  Widget build(BuildContext context) {
+    return new SmoothlyResizingOverflowBox(
+      size: _entries.last.childSize,
+      duration: config.duration,
+      curve: config.curve,
+      child: new Stack(_entries.map((_Entry entry) {
+        return new SizeObserver(
+          key: new ObjectKey(entry),
+          onSizeChanged: (Size newSize) {
+            setState(() {
+              entry.childSize = newSize;
+            });
+          },
+          child: entry.currentTransition
+        );
+      }).toList())
+    );
+  }
+}
diff --git a/packages/flutter/lib/widgets.dart b/packages/flutter/lib/widgets.dart
index 2ecb32b564..434ebf1258 100644
--- a/packages/flutter/lib/widgets.dart
+++ b/packages/flutter/lib/widgets.dart
@@ -11,6 +11,7 @@ export 'src/widgets/binding.dart';
 export 'src/widgets/dismissable.dart';
 export 'src/widgets/drag_target.dart';
 export 'src/widgets/editable_text.dart';
+export 'src/widgets/enter_exit_transition.dart';
 export 'src/widgets/focus.dart';
 export 'src/widgets/framework.dart';
 export 'src/widgets/gesture_detector.dart';