diff --git a/common/changes/@uifabric/experiments/AddPostReduceGrowCallbacks_2018-01-19-23-48.json b/common/changes/@uifabric/experiments/AddPostReduceGrowCallbacks_2018-01-19-23-48.json
new file mode 100644
index 0000000000000..2f6089719fc9d
--- /dev/null
+++ b/common/changes/@uifabric/experiments/AddPostReduceGrowCallbacks_2018-01-19-23-48.json
@@ -0,0 +1,11 @@
+{
+  "changes": [
+    {
+      "packageName": "@uifabric/experiments",
+      "comment": "Add callbacks for onDataReduced and onDataGrown",
+      "type": "minor"
+    }
+  ],
+  "packageName": "@uifabric/experiments",
+  "email": "staylo@microsoft.com"
+}
\ No newline at end of file
diff --git a/packages/experiments/src/components/CommandBar/CommandBar.base.tsx b/packages/experiments/src/components/CommandBar/CommandBar.base.tsx
index efb96a9b73c7a..9161424602709 100644
--- a/packages/experiments/src/components/CommandBar/CommandBar.base.tsx
+++ b/packages/experiments/src/components/CommandBar/CommandBar.base.tsx
@@ -155,7 +155,7 @@ export class CommandBarBase extends BaseComponent<ICommandBarProps, {}> implemen
 
   @autobind
   private _onReduceData(data: ICommandBarData): ICommandBarData | undefined {
-    const { endAligned } = this.props;
+    const { endAligned, onDataReduced } = this.props;
     let { primaryItems, overflowItems, cacheKey, farItems } = data;
 
     // Use first item if endAligned, otherwise use last item
@@ -168,6 +168,10 @@ export class CommandBarBase extends BaseComponent<ICommandBarProps, {}> implemen
       primaryItems = endAligned ? primaryItems.slice(1) : primaryItems.slice(0, -1);
       cacheKey = this._computeCacheKey(primaryItems, farItems!, !!overflowItems.length);
 
+      if (onDataReduced) {
+        onDataReduced(movedItem);
+      }
+
       return { ...data, primaryItems, overflowItems, cacheKey };
     }
 
@@ -176,7 +180,7 @@ export class CommandBarBase extends BaseComponent<ICommandBarProps, {}> implemen
 
   @autobind
   private _onGrowData(data: ICommandBarData): ICommandBarData | undefined {
-    const { endAligned } = this.props;
+    const { endAligned, onDataGrown } = this.props;
     let { primaryItems, overflowItems, cacheKey, minimumOverflowItems, farItems } = data;
     const movedItem = overflowItems[0];
 
@@ -189,6 +193,10 @@ export class CommandBarBase extends BaseComponent<ICommandBarProps, {}> implemen
       primaryItems = endAligned ? [movedItem, ...primaryItems] : [...primaryItems, movedItem];
       cacheKey = this._computeCacheKey(primaryItems, farItems!, !!overflowItems.length);
 
+      if (onDataGrown) {
+        onDataGrown(movedItem);
+      }
+
       return { ...data, primaryItems, overflowItems, cacheKey };
     }
 
diff --git a/packages/experiments/src/components/CommandBar/CommandBar.types.ts b/packages/experiments/src/components/CommandBar/CommandBar.types.ts
index 494f85ed0f4a9..53ead53555ebf 100644
--- a/packages/experiments/src/components/CommandBar/CommandBar.types.ts
+++ b/packages/experiments/src/components/CommandBar/CommandBar.types.ts
@@ -89,6 +89,16 @@ export interface ICommandBarProps extends React.HTMLAttributes<HTMLDivElement> {
    */
   onGrowData?: (data: ICommandBarData) => ICommandBarData;
 
+  /**
+   * Function callback invoked when data has been reduced.
+   */
+  onDataReduced?: (movedItem: ICommandBarItemProps) => void;
+
+  /**
+   * Function callback invoked when data has been grown.
+   */
+  onDataGrown?: (movedItem: ICommandBarItemProps) => void;
+
   /**
    * Additional css class to apply to the command bar
    * @defaultvalue undefined