diff --git a/src/utils/OrderedMap.js b/src/utils/OrderedMap.js
index 8c139263d7..57cfa54383 100644
--- a/src/utils/OrderedMap.js
+++ b/src/utils/OrderedMap.js
@@ -85,8 +85,6 @@ function OrderedMapImpl(normalizedObj, computedLength) {
  * Validates a "public" key - that is, one that the public facing API supplies.
  * The key is then normalized for internal storage. In order to be considered
  * valid, all keys must be non-empty, defined, non-null strings or numbers.
- * Since this already costs a function invocation, will avoid additional call to
- * `throwIf`.
  *
  * @param {string?} key Validates that the key is suitable for use in a
  * `OrderedMap`.
diff --git a/src/utils/Transaction.js b/src/utils/Transaction.js
index 8e630d88ab..861919d3d1 100644
--- a/src/utils/Transaction.js
+++ b/src/utils/Transaction.js
@@ -18,23 +18,7 @@
 
 "use strict";
 
-var throwIf = require('throwIf');
-
-var DUAL_TRANSACTION = 'DUAL_TRANSACTION';
-var MISSING_TRANSACTION = 'MISSING_TRANSACTION';
-if (__DEV__) {
-  DUAL_TRANSACTION =
-    'Cannot initialize transaction when there is already an outstanding ' +
-    'transaction. Common causes of this are trying to render a component ' +
-    'when you are already rendering a component or attempting a state ' +
-    'transition while in a render function. Another possibility is that ' +
-    'you are rendering new content (or state transitioning) in a ' +
-    'componentDidRender callback. If this is not the case, please report the ' +
-    'issue immediately.';
-
-  MISSING_TRANSACTION =
-    'Cannot close transaction when there is none open.';
-}
+var invariant = require('invariant');
 
 /**
  * `Transaction` creates a black box that is able to wrap any method such that
@@ -156,26 +140,33 @@ var Mixin = {
    * @return Return value from `method`.
    */
   perform: function(method, scope, a, b, c, d, e, f) {
-    throwIf(this.isInTransaction(), DUAL_TRANSACTION);
+    invariant(
+      !this.isInTransaction(),
+      'Transaction.perform(...): Cannot initialize a transaction when there ' +
+      'is already an outstanding transaction.'
+    );
     var memberStart = Date.now();
-    var err = null;
+    var errorToThrow = null;
     var ret;
     try {
       this.initializeAll();
       ret = method.call(scope, a, b, c, d, e, f);
-    } catch (ie_requires_catch) {
-      err = ie_requires_catch;
+    } catch (error) {
+      // IE8 requires `catch` in order to use `finally`.
+      errorToThrow = error;
     } finally {
       var memberEnd = Date.now();
       this.methodInvocationTime += (memberEnd - memberStart);
       try {
         this.closeAll();
-      } catch (closeAllErr) {
-        err = err || closeAllErr;
+      } catch (closeError) {
+        // If `method` throws, prefer to show that stack trace over any thrown
+        // by invoking `closeAll`.
+        errorToThrow = errorToThrow || closeError;
       }
     }
-    if (err) {
-      throw err;
+    if (errorToThrow) {
+      throw errorToThrow;
     }
     return ret;
   },
@@ -184,15 +175,17 @@ var Mixin = {
     this._isInTransaction = true;
     var transactionWrappers = this.transactionWrappers;
     var wrapperInitTimes = this.timingMetrics.wrapperInitTimes;
-    var err = null;
+    var errorToThrow = null;
     for (var i = 0; i < transactionWrappers.length; i++) {
       var initStart = Date.now();
       var wrapper = transactionWrappers[i];
       try {
-        this.wrapperInitData[i] =
-          wrapper.initialize ? wrapper.initialize.call(this) : null;
-      } catch (initErr) {
-        err = err || initErr;  // Remember the first error.
+        this.wrapperInitData[i] = wrapper.initialize ?
+          wrapper.initialize.call(this) :
+          null;
+      } catch (initError) {
+        // Prefer to show the stack trace of the first error.
+        errorToThrow = errorToThrow || initError;
         this.wrapperInitData[i] = Transaction.OBSERVED_ERROR;
       } finally {
         var curInitTime = wrapperInitTimes[i];
@@ -200,8 +193,8 @@ var Mixin = {
         wrapperInitTimes[i] = (curInitTime || 0) + (initEnd - initStart);
       }
     }
-    if (err) {
-      throw err;
+    if (errorToThrow) {
+      throw errorToThrow;
     }
   },
 
@@ -212,10 +205,13 @@ var Mixin = {
    * invoked).
    */
   closeAll: function() {
-    throwIf(!this.isInTransaction(), MISSING_TRANSACTION);
+    invariant(
+      this.isInTransaction(),
+      'Transaction.closeAll(): Cannot close transaction when none are open.'
+    );
     var transactionWrappers = this.transactionWrappers;
     var wrapperCloseTimes = this.timingMetrics.wrapperCloseTimes;
-    var err = null;
+    var errorToThrow = null;
     for (var i = 0; i < transactionWrappers.length; i++) {
       var wrapper = transactionWrappers[i];
       var closeStart = Date.now();
@@ -224,8 +220,9 @@ var Mixin = {
         if (initData !== Transaction.OBSERVED_ERROR) {
           wrapper.close && wrapper.close.call(this, initData);
         }
-      } catch (closeErr) {
-        err = err || closeErr;  // Remember the first error.
+      } catch (closeError) {
+        // Prefer to show the stack trace of the first error.
+        errorToThrow = errorToThrow || closeError;
       } finally {
         var closeEnd = Date.now();
         var curCloseTime = wrapperCloseTimes[i];
@@ -234,14 +231,16 @@ var Mixin = {
     }
     this.wrapperInitData.length = 0;
     this._isInTransaction = false;
-    if (err) {
-      throw err;
+    if (errorToThrow) {
+      throw errorToThrow;
     }
   }
 };
 
 var Transaction = {
+
   Mixin: Mixin,
+
   /**
    * Token to look for to determine if an error occured.
    */
diff --git a/src/utils/accumulate.js b/src/utils/accumulate.js
index 86f58b966a..7905f2bf9c 100644
--- a/src/utils/accumulate.js
+++ b/src/utils/accumulate.js
@@ -18,50 +18,34 @@
 
 "use strict";
 
-var throwIf = require('throwIf');
-
-var INVALID_ARGS = 'INVALID_ACCUM_ARGS';
-
-if (__DEV__) {
-  INVALID_ARGS =
-    'accumulate requires non empty (non-null, defined) next ' +
-    'values. All arrays accumulated must not contain any empty items.';
-}
+var invariant = require('invariant');
 
 /**
- * Accumulates items that must never be empty, into a result in a manner that
- * conserves memory - avoiding allocation of arrays until they are needed. The
- * accumulation may start and/or end up being a single element or an array
- * depending on the total count (if greater than one, an array is allocated).
- * Handles most common case first (starting with an empty current value and
- * acquiring one).
- * @return {Accumulation} An accumulation which is either a single item or an
- * Array of items.
+ * Accumulates items that must not be null or undefined.
+ *
+ * This is used to conserve memory by avoiding array allocations.
+ *
+ * @return {*|array<*>} An accumulation of items.
  */
-function accumulate(cur, next) {
-  var curValIsEmpty = cur == null;   // Will test for emptiness (null/undef)
-  var nextValIsEmpty = next === null;
-  if (__DEV__) {
-    throwIf(nextValIsEmpty, INVALID_ARGS);
-  }
-  if (nextValIsEmpty) {
-    return cur;
+function accumulate(current, next) {
+  invariant(
+    next != null,
+    'accumulate(...): Accumulated items must be not be null or undefined.'
+  );
+  if (current == null) {
+    return next;
   } else {
-    if (curValIsEmpty) {
-      return next;
+    // Both are not empty. Warning: Never call x.concat(y) when you are not
+    // certain that x is an Array (x could be a string with concat method).
+    var currentIsArray = Array.isArray(current);
+    var nextIsArray = Array.isArray(next);
+    if (currentIsArray) {
+      return current.concat(next);
     } else {
-      // Both are not empty. Warning: Never call x.concat(y) when you are not
-      // certain that x is an Array (x could be a string with concat method).
-      var curIsArray = Array.isArray(cur);
-      var nextIsArray = Array.isArray(next);
-      if (curIsArray) {
-        return cur.concat(next);
+      if (nextIsArray) {
+        return [current].concat(next);
       } else {
-        if (nextIsArray) {
-          return [cur].concat(next);
-        } else {
-          return [cur, next];
-        }
+        return [current, next];
       }
     }
   }
diff --git a/src/utils/escapeTextForBrowser.js b/src/utils/escapeTextForBrowser.js
index eba906b9ab..8e49d31e9e 100644
--- a/src/utils/escapeTextForBrowser.js
+++ b/src/utils/escapeTextForBrowser.js
@@ -14,19 +14,12 @@
  * limitations under the License.
  *
  * @providesModule escapeTextForBrowser
+ * @typechecks static-only
  */
 
 "use strict";
 
-var throwIf = require('throwIf');
-
-var ESCAPE_TYPE_ERR;
-
-if (__DEV__) {
-  ESCAPE_TYPE_ERR =
-    'The React core has attempted to escape content that is of a ' +
-    'mysterious type (object etc) Escaping only works on numbers and strings';
-}
+var invariant = require('invariant');
 
 var ESCAPE_LOOKUP = {
   "&": "&amp;",
@@ -41,13 +34,19 @@ function escaper(match) {
   return ESCAPE_LOOKUP[match];
 }
 
-var escapeTextForBrowser = function (text) {
+/**
+ * Escapes text to prevent scripting attacks.
+ *
+ * @param {number|string} text Text value to escape.
+ * @return {string} An escaped string.
+ */
+function escapeTextForBrowser(text) {
   var type = typeof text;
-  var invalid = type === 'object';
-  if (__DEV__) {
-    throwIf(invalid, ESCAPE_TYPE_ERR);
-  }
-  if (text === '' || invalid) {
+  invariant(
+    type !== 'object',
+    'escapeTextForBrowser(...): Attempted to escape an object.'
+  );
+  if (text === '') {
     return '';
   } else {
     if (type === 'string') {
@@ -56,6 +55,6 @@ var escapeTextForBrowser = function (text) {
       return (''+text).replace(/[&><"'\/]/g, escaper);
     }
   }
-};
+}
 
 module.exports = escapeTextForBrowser;
diff --git a/src/utils/flattenChildren.js b/src/utils/flattenChildren.js
index ba2d6bdf77..17b24ef220 100644
--- a/src/utils/flattenChildren.js
+++ b/src/utils/flattenChildren.js
@@ -18,7 +18,7 @@
 
 "use strict";
 
-var throwIf = require('throwIf');
+var invariant = require('invariant');
 var traverseAllChildren = require('traverseAllChildren');
 
 /**
@@ -27,14 +27,14 @@ var traverseAllChildren = require('traverseAllChildren');
  * @param {!string} name String name of key path to child.
  */
 function flattenSingleChildIntoContext(traverseContext, child, name) {
-  // We found a component instance
+  // We found a component instance.
   var result = traverseContext;
-  if (__DEV__) {
-    throwIf(
-      result.hasOwnProperty(name),
-      traverseAllChildren.DUPLICATE_KEY_ERROR
-    );
-  }
+  invariant(
+    !result.hasOwnProperty(name),
+    'flattenChildren(...): Encountered two children with the same key, `%s`. ' +
+    'Children keys must be unique.',
+    name
+  );
   result[name] = child;
 }
 
@@ -43,7 +43,7 @@ function flattenSingleChildIntoContext(traverseContext, child, name) {
  * @return {!object} flattened children keyed by name.
  */
 function flattenChildren(children) {
-  if (children === null || children === undefined) {
+  if (children == null) {
     return children;
   }
   var result = {};
diff --git a/src/utils/keyMirror.js b/src/utils/keyMirror.js
index bc64cabd9d..4e078e85a4 100644
--- a/src/utils/keyMirror.js
+++ b/src/utils/keyMirror.js
@@ -14,36 +14,38 @@
  * limitations under the License.
  *
  * @providesModule keyMirror
+ * @typechecks static-only
  */
 
 "use strict";
 
-var throwIf = require('throwIf');
-
-var NOT_OBJECT_ERROR = 'NOT_OBJECT_ERROR';
-if (__DEV__) {
-  NOT_OBJECT_ERROR = 'keyMirror only works on objects';
-}
+var invariant = require('invariant');
 
 /**
- * Utility for constructing enums with keys being equal to the associated
- * values, even when using advanced key crushing. This is useful for debugging,
- * but also for using the values themselves as lookups into the enum.
- * Example:
- * var COLORS = keyMirror({blue: null, red: null});
- * var myColor = COLORS.blue;
- * var isColorValid = !!COLORS[myColor]
+ * Constructs an enumeration with keys equal to their value.
+ *
+ * For example:
+ *
+ *   var COLORS = keyMirror({blue: null, red: null});
+ *   var myColor = COLORS.blue;
+ *   var isColorValid = !!COLORS[myColor];
+ *
  * The last line could not be performed if the values of the generated enum were
  * not equal to their keys.
- * Input:  {key1: val1, key2: val2}
- * Output: {key1: key1, key2: key2}
+ *
+ *   Input:  {key1: val1, key2: val2}
+ *   Output: {key1: key1, key2: key2}
+ *
+ * @param {object} obj
+ * @return {object}
  */
 var keyMirror = function(obj) {
   var ret = {};
   var key;
-
-  throwIf(!(obj instanceof Object) || Array.isArray(obj), NOT_OBJECT_ERROR);
-
+  invariant(
+    obj instanceof Object && !Array.isArray(obj),
+    'keyMirror(...): Argument must be an object.'
+  );
   for (key in obj) {
     if (!obj.hasOwnProperty(key)) {
       continue;
diff --git a/src/utils/mapAllChildren.js b/src/utils/mapAllChildren.js
index c035d9797d..9eff484798 100644
--- a/src/utils/mapAllChildren.js
+++ b/src/utils/mapAllChildren.js
@@ -20,7 +20,7 @@
 
 var PooledClass = require('PooledClass');
 
-var throwIf = require('throwIf');
+var invariant = require('invariant');
 var traverseAllChildren = require('traverseAllChildren');
 
 var threeArgumentPooler = PooledClass.threeArgumentPooler;
@@ -50,13 +50,13 @@ function mapSingleChildIntoContext(traverseContext, child, name, i) {
   var mapFunction = mapBookKeeping.mapFunction;
   var mapContext = mapBookKeeping.mapContext;
   var mappedChild = mapFunction.call(mapContext, child, name, i);
-  // We found a component instance
-  if (__DEV__) {
-    throwIf(
-      mapResult.hasOwnProperty(name),
-      traverseAllChildren.DUPLICATE_KEY_ERROR
-    );
-  }
+  // We found a component instance.
+  invariant(
+    !mapResult.hasOwnProperty(name),
+    'mapAllChildren(...): Encountered two children with the same key, `%s`. ' +
+    'Children keys must be unique.',
+    name
+  );
   mapResult[name] = mappedChild;
 }
 
diff --git a/src/utils/mergeDeepInto.js b/src/utils/mergeDeepInto.js
index 36131bb320..e86c95729f 100644
--- a/src/utils/mergeDeepInto.js
+++ b/src/utils/mergeDeepInto.js
@@ -21,9 +21,8 @@
 
 "use strict";
 
-var keyMirror = require('keyMirror');
+var invariant = require('invariant');
 var mergeHelpers = require('mergeHelpers');
-var throwIf = require('throwIf');
 
 var ArrayStrategies = mergeHelpers.ArrayStrategies;
 var checkArrayStrategy = mergeHelpers.checkArrayStrategy;
@@ -33,20 +32,6 @@ var checkMergeObjectArgs = mergeHelpers.checkMergeObjectArgs;
 var isTerminal = mergeHelpers.isTerminal;
 var normalizeMergeArg = mergeHelpers.normalizeMergeArg;
 
-var ERRORS = keyMirror({
-  RUN_TIME_ARRAY_MERGE_FAIL: null
-});
-
-if (__DEV__) {
-  ERRORS = {
-    RUN_TIME_ARRAY_MERGE_FAIL:
-      "The caller has not supplied an ArrayStrategy. This is supported as " +
-      "long as the data structures being merged do not contain two Arrays " +
-      "that must be merged together, which is exactly what has just " +
-      "happened. Change the call site to supply an Array merge resolver."
-  };
-}
-
 /**
  * Every deep merge function must handle merging in each of the following cases
  * at every level. We may refer to letters below in implementations. For each
@@ -161,9 +146,10 @@ var mergeSingleFieldDeep = function(one, two, key, arrayStrategy, level) {
     if (twoValIsTerminal) {             // [E]
       one[key] = twoVal;
     } else if (twoValIsArray) {         // [F]
-      throwIf(
-        !ArrayStrategies[arrayStrategy],
-        ERRORS.RUN_TIME_ARRAY_MERGE_FAIL
+      invariant(
+        ArrayStrategies[arrayStrategy],
+        'mergeDeepInto(...): Attempted to merge two arrays, but a valid ' +
+        'ArrayStrategy was not specified.'
       );
       // Else: At this point, the only other valid option is `IndexByIndex`
       if (arrayStrategy === ArrayStrategies.Clobber) {
@@ -234,7 +220,7 @@ var mergeSingleFieldDeep = function(one, two, key, arrayStrategy, level) {
  */
 var mergeDeepInto = function(one, twoParam, arrayStrategy) {
   var two = normalizeMergeArg(twoParam);
-  checkArrayStrategy(arrayStrategy);  // Will be checked twice, for now
+  checkArrayStrategy(arrayStrategy); // Will be checked twice, for now.
   mergeDeepIntoObjects(one, two, arrayStrategy, 0);
 };
 
diff --git a/src/utils/mergeHelpers.js b/src/utils/mergeHelpers.js
index 5b46ff5d8e..d25cfcff87 100644
--- a/src/utils/mergeHelpers.js
+++ b/src/utils/mergeHelpers.js
@@ -20,8 +20,8 @@
 
 "use strict";
 
+var invariant = require('invariant');
 var keyMirror = require('keyMirror');
-var throwIf = require('throwIf');
 
 /**
  * Maximum number of levels to traverse. Will catch circular structures.
@@ -29,41 +29,6 @@ var throwIf = require('throwIf');
  */
 var MAX_MERGE_DEPTH = 36;
 
-var ERRORS = keyMirror({
-  MERGE_ARRAY_FAIL: null,
-  MERGE_CORE_FAILURE: null,
-  MERGE_TYPE_USAGE_FAILURE: null,
-  MERGE_DEEP_MAX_LEVELS: null,
-  MERGE_DEEP_NO_ARR_STRATEGY: null
-});
-
-if (__DEV__) {
-  ERRORS = {
-    MERGE_ARRAY_FAIL:
-      'Unsupported type passed to a merge function. You may have passed a ' +
-      'structure that contains an array and the merge function does not know ' +
-      'how to merge arrays. ',
-
-    MERGE_CORE_FAILURE:
-      'Critical assumptions about the merge functions have been violated. ' +
-      'This is the fault of the merge functions themselves, not necessarily ' +
-      'the callers.',
-
-    MERGE_TYPE_USAGE_FAILURE:
-      'Calling merge function with invalid types. You may call merge ' +
-      'functions (non-array non-terminal) OR (null/undefined) arguments. ' +
-      'mergeInto functions have the same requirements but with an added ' +
-      'restriction that the first parameter must not be null/undefined.',
-
-    MERGE_DEEP_MAX_LEVELS:
-      'Maximum deep merge depth exceeded. You may attempting to merge ' +
-      'circular structures in an unsupported way.',
-    MERGE_DEEP_NO_ARR_STRATEGY:
-      'You must provide an array strategy to deep merge functions to ' +
-      'instruct the deep merge how to resolve merging two arrays.'
-  };
-}
-
 /**
  * We won't worry about edge cases like new String('x') or new Boolean(true).
  * Functions are considered terminals, and arrays are not.
@@ -99,9 +64,11 @@ var mergeHelpers = {
    * @param {*} two Array to merge from.
    */
   checkMergeArrayArgs: function(one, two) {
-    throwIf(
-      !Array.isArray(one) || !Array.isArray(two),
-      ERRORS.MERGE_CORE_FAILURE
+    invariant(
+      Array.isArray(one) && Array.isArray(two),
+      'Critical assumptions about the merge functions have been violated. ' +
+      'This is the fault of the merge functions themselves, not necessarily ' +
+      'the callers.'
     );
   },
 
@@ -118,7 +85,12 @@ var mergeHelpers = {
    * @param {*} arg
    */
   checkMergeObjectArg: function(arg) {
-    throwIf(isTerminal(arg) || Array.isArray(arg), ERRORS.MERGE_CORE_FAILURE);
+    invariant(
+      !isTerminal(arg) && !Array.isArray(arg),
+      'Critical assumptions about the merge functions have been violated. ' +
+      'This is the fault of the merge functions themselves, not necessarily ' +
+      'the callers.'
+    );
   },
 
   /**
@@ -128,19 +100,23 @@ var mergeHelpers = {
    * @param {number} Level of recursion to validate against maximum.
    */
   checkMergeLevel: function(level) {
-    throwIf(level >= MAX_MERGE_DEPTH, ERRORS.MERGE_DEEP_MAX_LEVELS);
+    invariant(
+      level < MAX_MERGE_DEPTH,
+      'Maximum deep merge depth exceeded. You may be attempting to merge ' +
+      'circular structures in an unsupported way.'
+    );
   },
 
   /**
-   * Checks that a merge was not given a circular object or an object that had
-   * too great of depth.
+   * Checks that the supplied merge strategy is valid.
    *
-   * @param {number} Level of recursion to validate against maximum.
+   * @param {string} Array merge strategy.
    */
   checkArrayStrategy: function(strategy) {
-    throwIf(
-      strategy !== undefined && !(strategy in mergeHelpers.ArrayStrategies),
-      ERRORS.MERGE_DEEP_NO_ARR_STRATEGY
+    invariant(
+      strategy === undefined || strategy in mergeHelpers.ArrayStrategies,
+      'You must provide an array strategy to deep merge functions to ' +
+      'instruct the deep merge how to resolve merging two arrays.'
     );
   },
 
@@ -154,9 +130,8 @@ var mergeHelpers = {
   ArrayStrategies: keyMirror({
     Clobber: true,
     IndexByIndex: true
-  }),
+  })
 
-  ERRORS: ERRORS
 };
 
 module.exports = mergeHelpers;
diff --git a/src/utils/traverseAllChildren.js b/src/utils/traverseAllChildren.js
index 9fa6f1881f..21dd506d84 100644
--- a/src/utils/traverseAllChildren.js
+++ b/src/utils/traverseAllChildren.js
@@ -21,22 +21,7 @@
 var ReactComponent = require('ReactComponent');
 var ReactTextComponent = require('ReactTextComponent');
 
-var throwIf = require('throwIf');
-
-/**
- * @polyFill Array.isArray
- */
-var DUPLICATE_KEY_ERROR = 'DUPLICATE_KEY_ERROR';
-var INVALID_CHILD = 'INVALID_CHILD';
-if (__DEV__) {
-  INVALID_CHILD =
-    'You may not pass a child of that type to a React component. It ' +
-    'is a common mistake to try to pass a standard browser DOM element ' +
-    'as a child of a React component.';
-  DUPLICATE_KEY_ERROR =
-    'You have two children with identical keys. Make sure that you set the ' +
-    '"key" property to a unique value such as a row ID.';
-}
+var invariant = require('invariant');
 
 /**
  * TODO: Test that:
@@ -88,7 +73,11 @@ var traverseAllChildrenImpl =
         subtreeCount = 1;
       } else {
         if (type === 'object') {
-          throwIf(children && children.nodeType === 1, INVALID_CHILD);
+          invariant(
+            children || children.nodeType !== 1,
+            'traverseAllChildren(...): Encountered an invalid child; DOM ' +
+            'elements are not valid children of React components.'
+          );
           for (var key in children) {
             if (children.hasOwnProperty(key)) {
               subtreeCount += traverseAllChildrenImpl(
@@ -135,10 +124,4 @@ function traverseAllChildren(children, callback, traverseContext) {
   }
 }
 
-/**
- * Export the error code for use in other walking/mapping code.
- */
-traverseAllChildren.DUPLICATE_KEY_ERROR = DUPLICATE_KEY_ERROR;
-
-
 module.exports = traverseAllChildren;