diff --git a/src/renderers/shared/fiber/ReactFiberBeginWork.js b/src/renderers/shared/fiber/ReactFiberBeginWork.js
index b75fe2e67be4..c199c8c921f7 100644
--- a/src/renderers/shared/fiber/ReactFiberBeginWork.js
+++ b/src/renderers/shared/fiber/ReactFiberBeginWork.js
@@ -40,9 +40,6 @@ var {
   NoWork,
   OffscreenPriority,
 } = require('ReactPriorityLevel');
-var {
-  mergeUpdateQueue,
-} = require('ReactFiberUpdateQueue');
 var {
   Placement,
 } = require('ReactTypeOfSideEffect');
@@ -51,7 +48,11 @@ var ReactFiberClassComponent = require('ReactFiberClassComponent');
 module.exports = function<T, P, I, TI, C>(config : HostConfig<T, P, I, TI, C>, scheduleUpdate : (fiber: Fiber, priorityLevel : PriorityLevel) => void) {
 
   const {
-    mount,
+    adoptClassInstance,
+    constructClassInstance,
+    mountClassInstance,
+    resumeMountClassInstance,
+    updateClassInstance,
   } = ReactFiberClassComponent(scheduleUpdate);
 
   function markChildAsProgressed(current, workInProgress, priorityLevel) {
@@ -156,54 +157,27 @@ module.exports = function<T, P, I, TI, C>(config : HostConfig<T, P, I, TI, C>, s
   }
 
   function updateClassComponent(current : ?Fiber, workInProgress : Fiber) {
-    // A class component update is the result of either new props or new state.
-    // Account for the possibly of missing pending props by falling back to the
-    // memoized props.
-    var props = workInProgress.pendingProps;
-    if (!props) {
-      // If there isn't any new props, then we'll reuse the memoized props.
-      // This could be from already completed work.
-      props = workInProgress.memoizedProps;
-      if (!props) {
-        throw new Error('There should always be pending or memoized props.');
+    let shouldUpdate;
+    if (!current) {
+      if (!workInProgress.stateNode) {
+        // In the initial pass we might need to construct the instance.
+        constructClassInstance(workInProgress);
+        mountClassInstance(workInProgress);
+        shouldUpdate = true;
+      } else {
+        // In a resume, we'll already have an instance we can reuse.
+        shouldUpdate = resumeMountClassInstance(workInProgress);
       }
-    }
-
-    // Compute the state using the memoized state and the update queue.
-    var updateQueue = workInProgress.updateQueue;
-    var previousState = workInProgress.memoizedState;
-    var state;
-    if (updateQueue) {
-      state = mergeUpdateQueue(updateQueue, previousState, props);
     } else {
-      state = previousState;
+      shouldUpdate = updateClassInstance(current, workInProgress);
     }
-
-    var instance = workInProgress.stateNode;
-    if (!instance) {
-      var ctor = workInProgress.type;
-      workInProgress.stateNode = instance = new ctor(props);
-      mount(workInProgress, instance);
-      state = instance.state || null;
-    } else if (typeof instance.shouldComponentUpdate === 'function' &&
-               !(updateQueue && updateQueue.isForced)) {
-      if (workInProgress.memoizedProps !== null) {
-        // Reset the props, in case this is a ping-pong case rather than a
-        // completed update case. For the completed update case, the instance
-        // props will already be the memoizedProps.
-        instance.props = workInProgress.memoizedProps;
-        instance.state = workInProgress.memoizedState;
-        if (!instance.shouldComponentUpdate(props, state)) {
-          return bailoutOnAlreadyFinishedWork(current, workInProgress);
-        }
-      }
+    if (!shouldUpdate) {
+      return bailoutOnAlreadyFinishedWork(current, workInProgress);
     }
-
-    instance.props = props;
-    instance.state = state;
-    var nextChildren = instance.render();
+    // Rerender
+    const instance = workInProgress.stateNode;
+    const nextChildren = instance.render();
     reconcileChildren(current, workInProgress, nextChildren);
-
     return workInProgress.child;
   }
 
@@ -258,22 +232,21 @@ module.exports = function<T, P, I, TI, C>(config : HostConfig<T, P, I, TI, C>, s
   }
 
   function mountIndeterminateComponent(current, workInProgress) {
+    if (current) {
+      throw new Error('An indeterminate component should never have mounted.');
+    }
     var fn = workInProgress.type;
     var props = workInProgress.pendingProps;
     var value = fn(props);
     if (typeof value === 'object' && value && typeof value.render === 'function') {
       // Proceed under the assumption that this is a class instance
       workInProgress.tag = ClassComponent;
-      if (current) {
-        current.tag = ClassComponent;
-      }
+      adoptClassInstance(workInProgress, value);
+      mountClassInstance(workInProgress);
       value = value.render();
     } else {
       // Proceed under the assumption that this is a functional component
       workInProgress.tag = FunctionalComponent;
-      if (current) {
-        current.tag = FunctionalComponent;
-      }
     }
     reconcileChildren(current, workInProgress, value);
     return workInProgress.child;
diff --git a/src/renderers/shared/fiber/ReactFiberClassComponent.js b/src/renderers/shared/fiber/ReactFiberClassComponent.js
index 6110057607cb..7392fbb6c90a 100644
--- a/src/renderers/shared/fiber/ReactFiberClassComponent.js
+++ b/src/renderers/shared/fiber/ReactFiberClassComponent.js
@@ -21,6 +21,7 @@ var {
   createUpdateQueue,
   addToQueue,
   addCallbackToQueue,
+  mergeUpdateQueue,
 } = require('ReactFiberUpdateQueue');
 var ReactInstanceMap = require('ReactInstanceMap');
 
@@ -70,20 +71,155 @@ module.exports = function(scheduleUpdate : (fiber: Fiber, priorityLevel : Priori
     },
   };
 
-  function mount(workInProgress : Fiber, instance : any) {
-    const state = instance.state || null;
-    // The initial state must be added to the update queue in case
-    // setState is called before the initial render.
-    if (state !== null) {
-      workInProgress.updateQueue = createUpdateQueue(state);
-    }
+  function adoptClassInstance(workInProgress : Fiber, instance : any) : void {
+    instance.updater = updater;
+    workInProgress.stateNode = instance;
     // The instance needs access to the fiber so that it can schedule updates
     ReactInstanceMap.set(instance, workInProgress);
-    instance.updater = updater;
+  }
+
+  function constructClassInstance(workInProgress : Fiber) : any {
+    const ctor = workInProgress.type;
+    const props = workInProgress.pendingProps;
+    const instance = new ctor(props);
+    adoptClassInstance(workInProgress, instance);
+    return instance;
+  }
+
+  // Invokes the mount life-cycles on a previously never rendered instance.
+  function mountClassInstance(workInProgress : Fiber) : void {
+    const instance = workInProgress.stateNode;
+
+    const state = instance.state || null;
+
+    // A class component update is the result of either new props or new state.
+    // Account for the possibly of missing pending props by falling back to the
+    // memoized props.
+    let props = workInProgress.pendingProps;
+    if (!props) {
+      throw new Error('There must be pending props for an initial mount.');
+    }
+
+    instance.props = props;
+    instance.state = state;
+
+    if (typeof instance.componentWillMount === 'function') {
+      instance.componentWillMount();
+      // If we had additional state updates during this life-cycle, let's
+      // process them now.
+      const updateQueue = workInProgress.updateQueue;
+      if (updateQueue) {
+        instance.state = mergeUpdateQueue(updateQueue, state, props);
+      }
+    }
+  }
+
+  // Called on a preexisting class instance. Returns false if a resumed render
+  // could be reused.
+  function resumeMountClassInstance(workInProgress : Fiber) : boolean {
+    const instance = workInProgress.stateNode;
+    let newState = workInProgress.memoizedState;
+    let newProps = workInProgress.pendingProps;
+    if (!newProps) {
+      // If there isn't any new props, then we'll reuse the memoized props.
+      // This could be from already completed work.
+      newProps = workInProgress.memoizedProps;
+      if (!newProps) {
+        throw new Error('There should always be pending or memoized props.');
+      }
+    }
+
+    // TODO: Should we deal with a setState that happened after the last
+    // componentWillMount and before this componentWillMount? Probably
+    // unsupported anyway.
+
+    const updateQueue = workInProgress.updateQueue;
+
+    // If this completed, we might be able to just reuse this instance.
+    if (typeof instance.shouldComponentUpdate === 'function' &&
+        !(updateQueue && updateQueue.isForced) &&
+        workInProgress.memoizedProps !== null &&
+        !instance.shouldComponentUpdate(newProps, newState)) {
+      return false;
+    }
+
+    // If we didn't bail out we need to construct a new instance. We don't
+    // want to reuse one that failed to fully mount.
+    const newInstance = constructClassInstance(workInProgress);
+    newInstance.props = newProps;
+    newInstance.state = newState = newInstance.state || null;
+
+    if (typeof newInstance.componentWillMount === 'function') {
+      newInstance.componentWillMount();
+      // If we had additional state updates during this life-cycle, let's
+      // process them now.
+      const newUpdateQueue = workInProgress.updateQueue;
+      if (newUpdateQueue) {
+        instance.state = mergeUpdateQueue(newUpdateQueue, newState, newProps);
+      }
+    }
+    return true;
+  }
+
+  // Invokes the update life-cycles and returns false if it shouldn't rerender.
+  function updateClassInstance(current : Fiber, workInProgress : Fiber) : boolean {
+    const instance = workInProgress.stateNode;
+
+    const oldProps = workInProgress.memoizedProps || current.memoizedProps;
+    let newProps = workInProgress.pendingProps;
+    if (!newProps) {
+      // If there aren't any new props, then we'll reuse the memoized props.
+      // This could be from already completed work.
+      newProps = oldProps;
+      if (!newProps) {
+        throw new Error('There should always be pending or memoized props.');
+      }
+    }
+
+    // Note: During these life-cycles, instance.props/instance.state are what
+    // ever the previously attempted to render - not the "current". However,
+    // during componentDidUpdate we pass the "current" props.
+
+    if (oldProps !== newProps) {
+      if (typeof instance.componentWillReceiveProps === 'function') {
+        instance.componentWillReceiveProps(newProps);
+      }
+    }
+
+    // Compute the next state using the memoized state and the update queue.
+    const updateQueue = workInProgress.updateQueue;
+    const previousState = workInProgress.memoizedState;
+    // TODO: Previous state can be null.
+    let newState;
+    if (updateQueue) {
+      newState = mergeUpdateQueue(updateQueue, previousState, newProps);
+    } else {
+      newState = previousState;
+    }
+
+    if (typeof instance.shouldComponentUpdate === 'function' &&
+        !(updateQueue && updateQueue.isForced) &&
+        oldProps !== null &&
+        !instance.shouldComponentUpdate(newProps, newState)) {
+      // TODO: Should this get the new props/state updated regardless?
+      return false;
+    }
+
+    if (typeof instance.componentWillUpdate === 'function') {
+      instance.componentWillUpdate(newProps, newState);
+    }
+
+    instance.props = newProps;
+    instance.state = newState;
+    return true;
   }
 
   return {
-    mount,
+    adoptClassInstance,
+    constructClassInstance,
+    mountClassInstance,
+    resumeMountClassInstance,
+    updateClassInstance,
   };
 
 };
diff --git a/src/renderers/shared/fiber/ReactFiberScheduler.js b/src/renderers/shared/fiber/ReactFiberScheduler.js
index 99c8682a17eb..e29729d8626f 100644
--- a/src/renderers/shared/fiber/ReactFiberScheduler.js
+++ b/src/renderers/shared/fiber/ReactFiberScheduler.js
@@ -79,14 +79,13 @@ module.exports = function<T, P, I, TI, C>(config : HostConfig<T, P, I, TI, C>) {
       }
       nextScheduledRoot = nextScheduledRoot.nextScheduledRoot;
     }
-    // TODO: This is scanning one root at a time. It should be scanning all
-    // roots for high priority work before moving on to lower priorities.
     let root = nextScheduledRoot;
     let highestPriorityRoot = null;
     let highestPriorityLevel = NoWork;
     while (root) {
-      if (highestPriorityLevel === NoWork ||
-          highestPriorityLevel > root.current.pendingWorkPriority) {
+      if (root.current.pendingWorkPriority !== NoWork && (
+          highestPriorityLevel === NoWork ||
+          highestPriorityLevel > root.current.pendingWorkPriority)) {
         highestPriorityLevel = root.current.pendingWorkPriority;
         highestPriorityRoot = root;
       }
diff --git a/src/renderers/shared/fiber/__tests__/ReactIncremental-test.js b/src/renderers/shared/fiber/__tests__/ReactIncremental-test.js
index aa6cc245541b..467965a8e937 100644
--- a/src/renderers/shared/fiber/__tests__/ReactIncremental-test.js
+++ b/src/renderers/shared/fiber/__tests__/ReactIncremental-test.js
@@ -625,10 +625,10 @@ describe('ReactIncremental', () => {
     // Normally shouldComponentUpdate->false is not enough to determine that we
     // can safely reuse the old props, but I think in this case it would be ok,
     // since it is a resume of already started work.
-    // Because of the above we can also not reuse the work of Bar because the
+    // Because of the above we can not reuse the work of Bar because the
     // rerender of Content will generate a new element which will mean we don't
     // auto-bail out from Bar.
-    expect(ops).toEqual(['Content', 'Bar', 'Middle']);
+    expect(ops).toEqual(['Bar', 'Middle']);
 
   });
 
@@ -839,8 +839,14 @@ describe('ReactIncremental', () => {
   it('can call sCU while resuming a partly mounted component', () => {
     var ops = [];
 
+    var instances = new Set();
+
     class Bar extends React.Component {
       state = { y: 'A' };
+      constructor() {
+        super();
+        instances.add(this);
+      }
       shouldComponentUpdate(newProps, newState) {
         return this.props.x !== newProps.x ||
                this.state.y !== newState.y;
@@ -855,20 +861,29 @@ describe('ReactIncremental', () => {
       ops.push('Foo');
       return [
         <Bar key="a" x="A" />,
-        <Bar key="b" x="B" />,
+        <Bar key="b" x={props.step === 0 ? 'B' : 'B2'} />,
         <Bar key="c" x="C" />,
+        <Bar key="d" x="D" />,
       ];
     }
 
-    ReactNoop.render(<Foo />);
-    ReactNoop.flushDeferredPri(30);
-    expect(ops).toEqual(['Foo', 'Bar:A', 'Bar:B']);
+    ReactNoop.render(<Foo step={0} />);
+    ReactNoop.flushDeferredPri(40);
+    expect(ops).toEqual(['Foo', 'Bar:A', 'Bar:B', 'Bar:C']);
+
+    expect(instances.size).toBe(3);
 
     ops = [];
 
-    ReactNoop.render(<Foo />);
-    ReactNoop.flushDeferredPri(40);
-    expect(ops).toEqual(['Foo', 'Bar:B', 'Bar:C']);
+    ReactNoop.render(<Foo step={1} />);
+    ReactNoop.flushDeferredPri(50);
+    // A completed and was reused. B completed but couldn't be reused because
+    // props differences. C didn't complete and therefore couldn't be reused.
+    // D never even started so it needed a new instance.
+    expect(ops).toEqual(['Foo', 'Bar:B2', 'Bar:C', 'Bar:D']);
+
+    // We expect each rerender to correspond to a new instance.
+    expect(instances.size).toBe(6);
   });
 
   it('gets new props when setting state on a partly updated component', () => {
@@ -927,4 +942,296 @@ describe('ReactIncremental', () => {
     expect(ops).toEqual(['Bar:A-1', 'Baz', 'Baz']);
   });
 
+  it('calls componentWillMount twice if the initial render is aborted', () => {
+    var ops = [];
+
+    class LifeCycle extends React.Component {
+      state = { x: this.props.x };
+      componentWillMount() {
+        ops.push('componentWillMount:' + this.state.x + '-' + this.props.x);
+      }
+      componentDidMount() {
+        ops.push('componentDidMount:' + this.state.x + '-' + this.props.x);
+      }
+      render() {
+        return <span />;
+      }
+    }
+
+    function Trail() {
+      ops.push('Trail');
+    }
+
+    function App(props) {
+      ops.push('App');
+      return (
+        <div>
+          <LifeCycle x={props.x} />
+          <Trail />
+        </div>
+      );
+    }
+
+    ReactNoop.render(<App x={0} />);
+    ReactNoop.flushDeferredPri(30);
+
+    expect(ops).toEqual([
+      'App',
+      'componentWillMount:0-0',
+    ]);
+
+    ops = [];
+
+    ReactNoop.render(<App x={1} />);
+    ReactNoop.flush();
+
+    expect(ops).toEqual([
+      'App',
+      'componentWillMount:1-1',
+      'Trail',
+      'componentDidMount:1-1',
+    ]);
+  });
+
+  it('calls componentWill* twice if an update render is aborted', () => {
+    var ops = [];
+
+    class LifeCycle extends React.Component {
+      componentWillMount() {
+        ops.push('componentWillMount:' + this.props.x);
+      }
+      componentDidMount() {
+        ops.push('componentDidMount:' + this.props.x);
+      }
+      componentWillReceiveProps(nextProps) {
+        ops.push('componentWillReceiveProps:' + this.props.x + '-' + nextProps.x);
+      }
+      shouldComponentUpdate(nextProps) {
+        ops.push('shouldComponentUpdate:' + this.props.x + '-' + nextProps.x);
+        return true;
+      }
+      componentWillUpdate(nextProps) {
+        ops.push('componentWillUpdate:' + this.props.x + '-' + nextProps.x);
+      }
+      componentDidUpdate(prevProps) {
+        ops.push('componentDidUpdate:' + this.props.x + '-' + prevProps.x);
+      }
+      render() {
+        ops.push('render:' + this.props.x);
+        return <span />;
+      }
+    }
+
+    function Sibling() {
+      // The sibling is used to confirm that we've completed the first child,
+      // but not yet flushed.
+      ops.push('Sibling');
+      return <span />;
+    }
+
+    function App(props) {
+      ops.push('App');
+
+      return [
+        <LifeCycle key="a" x={props.x} />,
+        <Sibling key="b" />,
+      ];
+    }
+
+    ReactNoop.render(<App x={0} />);
+    ReactNoop.flush();
+
+    expect(ops).toEqual([
+      'App',
+      'componentWillMount:0',
+      'render:0',
+      'Sibling',
+      'componentDidMount:0',
+    ]);
+
+    ops = [];
+
+    ReactNoop.render(<App x={1} />);
+    ReactNoop.flushDeferredPri(30);
+
+    expect(ops).toEqual([
+      'App',
+      'componentWillReceiveProps:0-1',
+      'shouldComponentUpdate:0-1',
+      'componentWillUpdate:0-1',
+      'render:1',
+      'Sibling',
+      // no componentDidUpdate
+    ]);
+
+    ops = [];
+
+    ReactNoop.render(<App x={2} />);
+    ReactNoop.flush();
+
+    expect(ops).toEqual([
+      'App',
+      'componentWillReceiveProps:1-2',
+      'shouldComponentUpdate:1-2',
+      'componentWillUpdate:1-2',
+      'render:2',
+      'Sibling',
+      // When componentDidUpdate finally gets called, it covers both updates.
+      'componentDidUpdate:2-0',
+    ]);
+  });
+
+  it('does not call componentWillReceiveProps for state-only updates', () => {
+    var ops = [];
+
+    var instances = [];
+
+    class LifeCycle extends React.Component {
+      state = { x: 0 };
+      tick() {
+        this.setState({
+          x: this.state.x + 1,
+        });
+      }
+      componentWillMount() {
+        instances.push(this);
+        ops.push('componentWillMount:' + this.state.x);
+      }
+      componentDidMount() {
+        ops.push('componentDidMount:' + this.state.x);
+      }
+      componentWillReceiveProps(nextProps) {
+        ops.push('componentWillReceiveProps');
+      }
+      shouldComponentUpdate(nextProps, nextState) {
+        ops.push('shouldComponentUpdate:' + this.state.x + '-' + nextState.x);
+        return true;
+      }
+      componentWillUpdate(nextProps, nextState) {
+        ops.push('componentWillUpdate:' + this.state.x + '-' + nextState.x);
+      }
+      componentDidUpdate(prevProps, prevState) {
+        ops.push('componentDidUpdate:' + this.state.x + '-' + prevState.x);
+      }
+      render() {
+        ops.push('render:' + this.state.x);
+        return <span />;
+      }
+    }
+
+    // This wrap is a bit contrived because we can't pause a completed root and
+    // there is currently an issue where a component can't reuse its render
+    // output unless it fully completed.
+    class Wrap extends React.Component {
+      state = { y: 0 };
+      componentWillMount() {
+        instances.push(this);
+      }
+      tick() {
+        this.setState({
+          y: this.state.y + 1,
+        });
+      }
+      render() {
+        ops.push('Wrap');
+        return <LifeCycle y={this.state.y} />;
+      }
+    }
+
+    function Sibling() {
+      // The sibling is used to confirm that we've completed the first child,
+      // but not yet flushed.
+      ops.push('Sibling');
+      return <span />;
+    }
+
+    function App(props) {
+      ops.push('App');
+      return [
+        <Wrap key="a" />,
+        <Sibling key="b" />,
+      ];
+    }
+
+    ReactNoop.render(<App y={0} />);
+    ReactNoop.flush();
+
+    expect(ops).toEqual([
+      'App',
+      'Wrap',
+      'componentWillMount:0',
+      'render:0',
+      'Sibling',
+      'componentDidMount:0',
+    ]);
+
+    ops = [];
+
+    // LifeCycle
+    instances[1].tick();
+
+    ReactNoop.flushDeferredPri(25);
+
+    expect(ops).toEqual([
+      // no componentWillReceiveProps
+      'shouldComponentUpdate:0-1',
+      'componentWillUpdate:0-1',
+      'render:1',
+      // no componentDidUpdate
+    ]);
+
+    ops = [];
+
+    // LifeCycle
+    instances[1].tick();
+
+    ReactNoop.flush();
+
+    expect(ops).toEqual([
+      // no componentWillReceiveProps
+      'shouldComponentUpdate:1-2',
+      'componentWillUpdate:1-2',
+      'render:2',
+      // When componentDidUpdate finally gets called, it covers both updates.
+      'componentDidUpdate:2-0',
+    ]);
+
+    ops = [];
+
+    // Next we will update props of LifeCycle by updating its parent.
+
+    instances[0].tick();
+
+    ReactNoop.flushDeferredPri(30);
+
+    expect(ops).toEqual([
+      'Wrap',
+      'componentWillReceiveProps',
+      'shouldComponentUpdate:2-2',
+      'componentWillUpdate:2-2',
+      'render:2',
+      // no componentDidUpdate
+    ]);
+
+    ops = [];
+
+    // Next we will update LifeCycle directly but not with new props.
+    instances[1].tick();
+
+    ReactNoop.flush();
+
+    expect(ops).toEqual([
+      // This should not trigger another componentWillReceiveProps because
+      // we never got new props.
+      'shouldComponentUpdate:2-3',
+      'componentWillUpdate:2-3',
+      'render:3',
+      'componentDidUpdate:3-2',
+    ]);
+
+    // TODO: Test that we get the expected values for the same scenario with
+    // incomplete parents.
+
+  });
+
 });
diff --git a/src/renderers/shared/fiber/__tests__/ReactIncrementalSideEffects-test.js b/src/renderers/shared/fiber/__tests__/ReactIncrementalSideEffects-test.js
index 81a1e1c09321..897422d2274d 100644
--- a/src/renderers/shared/fiber/__tests__/ReactIncrementalSideEffects-test.js
+++ b/src/renderers/shared/fiber/__tests__/ReactIncrementalSideEffects-test.js
@@ -598,7 +598,7 @@ describe('ReactIncrementalSideEffects', () => {
       ),
     ]);
 
-    expect(ops).toEqual(['Baz', 'Bar', 'Baz', 'Bar', 'Bar']);
+    expect(ops).toEqual(['Bar', 'Baz', 'Bar', 'Bar']);
   });
 
   it('deprioritizes setStates that happens within a deprioritized tree', () => {