diff --git a/src/renderers/shared/fiber/ReactFiber.js b/src/renderers/shared/fiber/ReactFiber.js
index 86e5cbaed1..9961d7a1bc 100644
--- a/src/renderers/shared/fiber/ReactFiber.js
+++ b/src/renderers/shared/fiber/ReactFiber.js
@@ -12,14 +12,17 @@
 
 'use strict';
 
-var ReactTypesOfWork = require('ReactTypesOfWork');
+import type { TypeOfWork } from 'ReactTypeOfWork';
+import type { PriorityLevel } from 'ReactPriorityLevel';
+
+var ReactTypeOfWork = require('ReactTypeOfWork');
 var {
   IndeterminateComponent,
   ClassComponent,
   HostComponent,
   CoroutineComponent,
   YieldComponent,
-} = ReactTypesOfWork;
+} = ReactTypeOfWork;
 
 var ReactElement = require('ReactElement');
 
@@ -32,7 +35,7 @@ import type { ReactCoroutine, ReactYield } from 'ReactCoroutine';
 type Instance = {
 
   // Tag identifying the type of fiber.
-  tag: number,
+  tag: TypeOfWork,
 
   // The parent Fiber used to create this one. The type is constrained to the
   // Instance part of the Fiber since it is not safe to traverse the tree from
@@ -71,7 +74,7 @@ export type Fiber = Instance & {
   output: any, // This type will be more specific once we overload the tag.
 
   // This will be used to quickly determine if a subtree has no pending changes.
-  hasPendingChanges: bool,
+  pendingWorkPriority: PriorityLevel,
 
   // This is a pooled version of a Fiber. Every fiber that gets updated will
   // eventually have a pair. There are cases when we can clean up pairs to save
@@ -80,7 +83,7 @@ export type Fiber = Instance & {
 
 };
 
-var createFiber = function(tag : number, key : null | string) : Fiber {
+var createFiber = function(tag : TypeOfWork, key : null | string) : Fiber {
   return {
 
     // Instance
@@ -106,7 +109,7 @@ var createFiber = function(tag : number, key : null | string) : Fiber {
     memoizedInput: null,
     output: null,
 
-    hasPendingChanges: true,
+    pendingWorkPriority: 0,
 
     alternate: null,
 
diff --git a/src/renderers/shared/fiber/ReactFiberBeginWork.js b/src/renderers/shared/fiber/ReactFiberBeginWork.js
index 24ec08db90..0035d37e25 100644
--- a/src/renderers/shared/fiber/ReactFiberBeginWork.js
+++ b/src/renderers/shared/fiber/ReactFiberBeginWork.js
@@ -16,7 +16,7 @@ import type { ReactCoroutine } from 'ReactCoroutine';
 import type { Fiber } from 'ReactFiber';
 
 var ReactChildFiber = require('ReactChildFiber');
-var ReactTypesOfWork = require('ReactTypesOfWork');
+var ReactTypeOfWork = require('ReactTypeOfWork');
 var {
   IndeterminateComponent,
   FunctionalComponent,
@@ -25,7 +25,7 @@ var {
   CoroutineComponent,
   CoroutineHandlerPhase,
   YieldComponent,
-} = ReactTypesOfWork;
+} = ReactTypeOfWork;
 
 function reconcileChildren(current, workInProgress, nextChildren) {
   workInProgress.child = ReactChildFiber.reconcileChildFibers(
diff --git a/src/renderers/shared/fiber/ReactFiberCompleteWork.js b/src/renderers/shared/fiber/ReactFiberCompleteWork.js
index 179e050848..052331cb2e 100644
--- a/src/renderers/shared/fiber/ReactFiberCompleteWork.js
+++ b/src/renderers/shared/fiber/ReactFiberCompleteWork.js
@@ -18,7 +18,7 @@ import type { Fiber } from 'ReactFiber';
 import type { ReifiedYield } from 'ReactReifiedYield';
 
 var ReactChildFiber = require('ReactChildFiber');
-var ReactTypesOfWork = require('ReactTypesOfWork');
+var ReactTypeOfWork = require('ReactTypeOfWork');
 var {
   IndeterminateComponent,
   FunctionalComponent,
@@ -27,7 +27,7 @@ var {
   CoroutineComponent,
   CoroutineHandlerPhase,
   YieldComponent,
-} = ReactTypesOfWork;
+} = ReactTypeOfWork;
 
 function transferOutput(child : ?Fiber, parent : Fiber) {
   // If we have a single result, we just pass that through as the output to
diff --git a/src/renderers/shared/fiber/ReactPriorityLevel.js b/src/renderers/shared/fiber/ReactPriorityLevel.js
new file mode 100644
index 0000000000..b6930c2900
--- /dev/null
+++ b/src/renderers/shared/fiber/ReactPriorityLevel.js
@@ -0,0 +1,24 @@
+/**
+ * Copyright 2013-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ *
+ * @providesModule ReactPriorityLevel
+ * @flow
+ */
+
+'use strict';
+
+export type PriorityLevel = 0 | 1 | 2 | 3 | 4 | 5;
+
+module.exports = {
+  NoWork: 0, // No work is pending.
+  SynchronousPriority: 1, // For controlled text inputs. Synchronous side-effects.
+  AnimationPriority: 2, // Needs to complete before the next frame.
+  HighPriority: 3, // Interaction that needs to complete pretty soon to feel responsive.
+  LowPriority: 4, // Data fetching, or result from updating stores.
+  OffscreenPriority: 5, // Won't be visible but do the work in case it becomes visible.
+};
diff --git a/src/renderers/shared/fiber/ReactTypesOfWork.js b/src/renderers/shared/fiber/ReactTypeOfWork.js
similarity index 84%
rename from src/renderers/shared/fiber/ReactTypesOfWork.js
rename to src/renderers/shared/fiber/ReactTypeOfWork.js
index d54dc16807..915084416f 100644
--- a/src/renderers/shared/fiber/ReactTypesOfWork.js
+++ b/src/renderers/shared/fiber/ReactTypeOfWork.js
@@ -6,13 +6,15 @@
  * LICENSE file in the root directory of this source tree. An additional grant
  * of patent rights can be found in the PATENTS file in the same directory.
  *
- * @providesModule ReactTypesOfWork
+ * @providesModule ReactTypeOfWork
  * @flow
  */
 
 'use strict';
 
-var TypesOfWork = {
+export type TypeOfWork = 0 | 1 | 2 | 3 | 4 | 5 | 6;
+
+module.exports = {
   IndeterminateComponent: 0, // Before we know whether it is functional or class
   FunctionalComponent: 1,
   ClassComponent: 2,
@@ -21,5 +23,3 @@ var TypesOfWork = {
   CoroutineHandlerPhase: 5,
   YieldComponent: 6,
 };
-
-module.exports = TypesOfWork;