Index: /CKEditor/trunk/_source/core/command.js
===================================================================
--- /CKEditor/trunk/_source/core/command.js	(revision 3085)
+++ /CKEditor/trunk/_source/core/command.js	(revision 3086)
@@ -10,5 +10,8 @@
 	this.exec = function( data )
 	{
-		return commandDefinition.exec.call( this, editor, data );
+		if ( this.state == CKEDITOR.TRISTATE_DISABLED )
+			return false;
+
+		return ( commandDefinition.exec.call( this, editor, data ) !== false );
 	};
 
Index: /CKEditor/trunk/_source/core/commanddefinition.js
===================================================================
--- /CKEditor/trunk/_source/core/commanddefinition.js	(revision 3085)
+++ /CKEditor/trunk/_source/core/commanddefinition.js	(revision 3086)
@@ -25,4 +25,6 @@
  * @param {CKEDITOR.editor} editor The editor within which run the command.
  * @param {Object} [data] Additional data to be used to execute the command.
+ * @returns {Boolean} Whether the command has been successfully executed.
+ *		Defaults to "true", if nothing is returned.
  * @example
  * editorInstance.addCommand( 'sample',
@@ -34,2 +36,19 @@
  * });
  */
+
+/**
+ * Whether the command need to be hooked into the redo/undo system.
+ * @name  CKEDITOR.commandDefinition.canUndo
+ * @type {Boolean} If not defined or 'true' both hook into undo system, set it
+ *		to 'false' explicitly  keep it out.
+ * @field
+ * @example
+ * editorInstance.addCommand( 'alertName',
+ * {
+ *     exec : function( editor )
+ *     {
+ *         alert( editor.name );
+ *     },
+ *     canUndo : false    // No support for undo/redo
+ * });
+ */
Index: /CKEditor/trunk/_source/core/config.js
===================================================================
--- /CKEditor/trunk/_source/core/config.js	(revision 3085)
+++ /CKEditor/trunk/_source/core/config.js	(revision 3086)
@@ -148,5 +148,5 @@
 	 */
 
-	plugins : 'basicstyles,blockquote,button,clipboard,elementspath,horizontalrule,htmldataprocessor,image,indent,justify,keystrokes,link,list,newpage,pagebreak,pastefromword,pastetext,preview,print,removeformat,smiley,sourcearea,table,specialchar,tab,toolbar,wysiwygarea',
+	plugins : 'basicstyles,blockquote,button,clipboard,elementspath,horizontalrule,htmldataprocessor,image,indent,justify,keystrokes,link,list,newpage,pagebreak,pastefromword,pastetext,preview,print,removeformat,smiley,sourcearea,table,specialchar,tab,toolbar,undo,wysiwygarea',
 
 	/**
Index: /CKEditor/trunk/_source/core/dom/domobject.js
===================================================================
--- /CKEditor/trunk/_source/core/dom/domobject.js	(revision 3085)
+++ /CKEditor/trunk/_source/core/dom/domobject.js	(revision 3086)
@@ -65,5 +65,11 @@
 
 			// Get the listeners holder object.
-			var nativeListeners = this.getCustomData( '_cke_nativeListeners' ) || this.setCustomData( '_cke_nativeListeners', {} );
+			var nativeListeners = this.getCustomData( '_cke_nativeListeners' );
+
+			if ( !nativeListeners )
+			{
+				nativeListeners = {};
+				this.setCustomData( '_cke_nativeListeners', nativeListeners );
+			}
 
 			// Check if we have a listener for that event.
Index: /CKEditor/trunk/_source/core/editor.js
===================================================================
--- /CKEditor/trunk/_source/core/editor.js	(revision 3085)
+++ /CKEditor/trunk/_source/core/editor.js	(revision 3086)
@@ -387,6 +387,22 @@
 		{
 			var command = this.getCommand( commandName );
+
+			var eventData =
+			{
+				name: commandName,
+				commandData: data,
+				command: command
+			};
+
 			if ( command && command.state != CKEDITOR.TRISTATE_DISABLED )
-				return command.exec( data );
+			{
+				if ( this.fire( 'beforeCommandExec', eventData ) !== true )
+				{
+					eventData.returnValue = command.exec( eventData.commandData );
+
+					if ( this.fire( 'afterCommandExec', eventData ) !== true )
+						return eventData.returnValue;
+				}
+			}
 
 			// throw 'Unknown command name "' + commandName + '"';
@@ -455,4 +471,9 @@
 		},
 
+		loadSnapshot : function( snapshot )
+		{
+			this.fire( 'loadSnapshot', snapshot );
+		},
+
 		/**
 		 * Sets the editor data. The data must be provided in raw format.
Index: /CKEditor/trunk/_source/core/tools.js
===================================================================
--- /CKEditor/trunk/_source/core/tools.js	(revision 3085)
+++ /CKEditor/trunk/_source/core/tools.js	(revision 3086)
@@ -16,4 +16,21 @@
 CKEDITOR.tools =
 {
+	arrayCompare : function( arrayA, arrayB )
+	{
+		if ( !arrayA && !arrayB )
+			return true;
+
+		if ( !arrayA || !arrayB || arrayA.length != arrayB.length )
+			return false;
+
+		for ( var i = 0 ; i < arrayA.length ; i++ )
+		{
+			if ( arrayA[ i ] != arrayB[ i ] )
+				return false;
+		}
+
+		return true;
+	},
+
 	/**
 	 * Copy the properties from one object to another. By default, properties
Index: /CKEditor/trunk/_source/lang/en.js
===================================================================
--- /CKEditor/trunk/_source/lang/en.js	(revision 3085)
+++ /CKEditor/trunk/_source/lang/en.js	(revision 3086)
@@ -47,4 +47,6 @@
 	pagebreak		: 'Insert Page Break',
 	unlink			: 'Unlink',
+	undo			: 'Undo',
+	redo			: 'Redo',
 
 	// Common messages and labels.
Index: /CKEditor/trunk/_source/plugins/editingblock/plugin.js
===================================================================
--- /CKEditor/trunk/_source/plugins/editingblock/plugin.js	(revision 3085)
+++ /CKEditor/trunk/_source/plugins/editingblock/plugin.js	(revision 3086)
@@ -84,4 +84,10 @@
 					if ( editor.mode )
 						event.data = getMode( editor ).getSnapshotData();
+				});
+
+			editor.on( 'loadSnapshot', function( event )
+				{
+					if ( editor.mode )
+						getMode( editor ).loadSnapshotData( event.data );
 				});
 		}
Index: /CKEditor/trunk/_source/plugins/selection/plugin.js
===================================================================
--- /CKEditor/trunk/_source/plugins/selection/plugin.js	(revision 3085)
+++ /CKEditor/trunk/_source/plugins/selection/plugin.js	(revision 3086)
@@ -634,4 +634,15 @@
 		},
 
+		createBookmarks2 : function( normalized )
+		{
+			var bookmarks = [],
+				ranges = this.getRanges();
+
+			for ( var i = 0 ; i < ranges.length ; i++ )
+				bookmarks.push( ranges[i].createBookmark2( normalized ) );
+			
+			return bookmarks;
+		},
+
 		selectBookmarks : function( bookmarks )
 		{
Index: /CKEditor/trunk/_source/plugins/toolbar/plugin.js
===================================================================
--- /CKEditor/trunk/_source/plugins/toolbar/plugin.js	(revision 3085)
+++ /CKEditor/trunk/_source/plugins/toolbar/plugin.js	(revision 3086)
@@ -210,4 +210,5 @@
 		'NewPage', 'Preview', 'Print', '-',
 		'Cut', 'Copy', 'Paste', 'PasteText', 'PasteFromWord', '-',
+		'Undo', 'Redo', '-',
 		'Bold', 'Italic', 'Underline', 'Strike', '-',
 		'NumberedList', 'BulletedList', '-',
Index: /CKEditor/trunk/_source/plugins/undo/plugin.js
===================================================================
--- /CKEditor/trunk/_source/plugins/undo/plugin.js	(revision 3086)
+++ /CKEditor/trunk/_source/plugins/undo/plugin.js	(revision 3086)
@@ -0,0 +1,383 @@
+/*
+Copyright (c) 2003-2009, CKSource - Frederico Knabben. All rights reserved.
+For licensing, see LICENSE.html or http://ckeditor.com/license
+*/
+
+/**
+ * @fileOverview Undo/Redo system for saving shapshot for document modification
+ *		and other recordable changes.
+ */
+
+(function()
+{
+	CKEDITOR.plugins.add( 'undo',
+	{
+		requires : [ 'selection', 'wysiwygarea' ],
+
+		init : function( editor )
+		{
+			var undoManager = new UndoManager( editor );
+
+			var undoCommand = editor.addCommand( 'undo',
+				{
+					exec : function()
+					{
+						if ( undoManager.undo() )
+						{
+							editor.selectionChange();
+							this.fire( 'afterUndo' );
+						}
+					},
+					state : CKEDITOR.TRISTATE_DISABLED,
+					canUndo : false
+				});
+
+			var redoCommand = editor.addCommand( 'redo',
+				{
+					exec : function()
+					{
+						if ( undoManager.redo() )
+						{
+							editor.selectionChange();
+							this.fire( 'afterRedo' );
+						}
+					},
+					state : CKEDITOR.TRISTATE_DISABLED,
+					canUndo : false
+				});
+
+			undoManager.onChange = function()
+			{
+				undoCommand.setState( undoManager.undoable() ? CKEDITOR.TRISTATE_OFF : CKEDITOR.TRISTATE_DISABLED );
+				redoCommand.setState( undoManager.redoable() ? CKEDITOR.TRISTATE_OFF : CKEDITOR.TRISTATE_DISABLED );
+			};
+
+			function recordCommand( event )
+			{
+				// If the command hasn't been marked to not support undo.
+				if ( undoManager.enabled && event.data.command.canUndo !== false )
+					undoManager.save();
+			}
+
+			// We'll save snapshots before and after executing a command.
+			editor.on( 'beforeCommandExec', recordCommand );
+			editor.on( 'afterCommandExec', recordCommand );
+
+			// Make the undo manager available only in wysiwyg mode.
+			editor.on( 'mode', function()
+				{
+					if ( !undoManager.enabled && editor.mode == 'wysiwyg' )
+					{
+						undoManager.enabled = true;
+
+						editor.document.on( 'keydown', function( event )
+							{
+								// Do not capture CTRL hotkeys.
+								if ( !event.data.$.ctrlKey && !event.data.$.metaKey )
+									undoManager.type();
+							});
+
+						// Being this the first call, let's get an undo snapshot.
+						if ( undoManager.index == -1 )
+							undoManager.save();
+					}
+
+					undoManager.onChange();
+				});
+
+			editor.ui.addButton( 'Undo',
+				{
+					label : editor.lang.undo,
+					command : 'undo'
+				});
+
+			editor.ui.addButton( 'Redo',
+				{
+					label : editor.lang.redo,
+					command : 'redo'
+				});
+		}
+	});
+
+	// Gets a snapshot image which represent the current document status.
+	function Image( editor )
+	{
+		var selection = editor.getSelection();
+
+		this.contents	= editor.getSnapshot();
+		this.bookmarks	= selection && selection.createBookmarks2( true );
+		
+		// In IE, we need to remove the expando attributes.
+		if ( CKEDITOR.env.ie )
+			this.contents = this.contents.replace( /\s+_cke_expando=".*?"/g, '' );
+	}
+
+	Image.prototype =
+	{
+		equals : function( otherImage, contentOnly )
+		{
+			if ( this.contents != otherImage.contents )
+				return false;
+
+			if ( contentOnly )
+				return true;
+
+			var bookmarksA = this.bookmarks,
+				bookmarksB = otherImage.bookmarks;
+
+			if ( bookmarksA || bookmarksB )
+			{
+				if ( !bookmarksA || !bookmarksB || bookmarksA.length != bookmarksB.length )
+					return false;
+
+				for ( var i = 0 ; i < bookmarksA.length ; i++ )
+				{
+					var bookmarkA = bookmarksA[ i ],
+						bookmarkB = bookmarksB[ i ];
+
+					if (
+						bookmarkA.startOffset != bookmarkB.startOffset ||
+						bookmarkA.endOffset != bookmarkB.endOffset ||
+						!CKEDITOR.tools.arrayCompare( bookmarkA.start, bookmarkB.start ) ||
+						!CKEDITOR.tools.arrayCompare( bookmarkA.end, bookmarkB.end ) )
+					{
+						return false;
+					}
+				}
+			}
+
+			return true;
+		}
+	};
+
+	/**
+	 * @constructor Main logic for Redo/Undo feature.
+	 */
+	function UndoManager( editor )
+	{
+		this.typesCount = 0;
+
+		this.editor = editor;
+
+		/**
+		 * Stack for all the undo and redo snapshots, they're always created/removed
+		 * in consistency.
+		 */
+		this.snapshots = [];
+
+		/**
+		 * Current snapshot history index.
+		 */
+		this.index = -1;
+
+		this.limit = editor.config.undoStackSize;
+	}
+
+	UndoManager.prototype =
+	{
+		type : function()
+		{
+			if ( !this.typing )
+			{
+				var beforeTypeImage = new Image( this.editor );
+
+				// Use setTimeout, so we give the necessary time to the
+				// browser to insert the character into the DOM.
+				CKEDITOR.tools.setTimeout( function()
+					{
+						var currentSnapshot = this.editor.getSnapshot();
+						
+						// In IE, we need to remove the expando attributes.
+						if ( CKEDITOR.env.ie )
+							currentSnapshot = currentSnapshot.replace( /\s+_cke_expando=".*?"/g, '' );
+						
+						if ( beforeTypeImage.contents != currentSnapshot )
+						{
+							if ( !this.save( false, beforeTypeImage ) )
+							{
+								// Drop future snapshots.
+								this.snapshots.splice( this.index + 1, this.snapshots.length - this.index - 1 );
+							}
+
+							this.hasUndo = true;
+							this.hasRedo = false;
+
+							this.typesCount = 1;
+							this.typing = true;
+
+							this.onChange();
+						}
+					},
+					0, this );
+			
+				return;
+			}
+
+			this.typesCount++;
+
+			if ( this.typesCount > 25 )
+			{
+				this.save();
+				this.typesCount = 1;
+			}
+
+			this.typing = true;
+		},
+
+		fireChange : function()
+		{
+			this.hasUndo = !!this.getNextImage( true );
+			this.hasRedo = !!this.getNextImage( false );
+
+			this.typing = false;
+			this.typesCount = 0;
+
+			this.onChange();
+		},
+
+		/**
+		 * Save a snapshot of document image for later retrieve.
+		 */
+		save : function( onContentOnly, image )
+		{
+			var snapshots = this.snapshots;
+
+			// Get a content image.
+			if ( !image )
+				image = new Image( this.editor );
+
+			// Check if this is a duplicate. In such case, do nothing.
+			if ( this.currentImage && image.equals( this.currentImage, onContentOnly ) )
+				return false;
+
+			// Drop future snapshots.
+			snapshots.splice( this.index + 1, snapshots.length - this.index - 1 );
+
+			// If we have reached the limit, remove the oldest one.
+			if ( snapshots.length == this.limit )
+				snapshots.shift();
+
+			// Add the new image, updating the current index.
+			this.index = snapshots.push( image ) - 1;
+
+			this.currentImage = image;
+
+			this.fireChange();
+
+			return true;
+		},
+
+		restoreImage : function( image )
+		{
+			this.editor.loadSnapshot( image.contents );
+			
+			if ( image.bookmarks )
+				this.editor.getSelection().selectBookmarks( image.bookmarks );
+
+			this.index = image.index;
+
+			this.currentImage = image;
+
+			this.fireChange();
+		},
+
+		// Get the closest available image.
+		getNextImage : function( isUndo )
+		{
+			var snapshots = this.snapshots,
+				currentImage = this.currentImage,
+				image, i;
+
+			if ( currentImage )
+			{
+				if ( isUndo )
+				{
+					for ( i = this.index - 1 ; i >= 0 ; i-- )
+					{
+						image = snapshots[ i ];
+						if ( !currentImage.equals( image, true ) )
+						{
+							image.index = i;
+							return image;
+						}
+					}
+				}
+				else
+				{
+					for ( i = this.index + 1 ; i < snapshots.length ; i++ )
+					{
+						image = snapshots[ i ];
+						if ( !currentImage.equals( image, true ) )
+						{
+							image.index = i;
+							return image;
+						}
+					}
+				}
+			}
+
+			return null;
+		},
+
+		/**
+		 * Check the current redo state.
+		 * @return {Boolean} Whether the document has previous state to
+		 *		retrieve.
+		 */
+		redoable : function()
+		{
+			return this.enabled && this.hasRedo;
+		},
+
+		/**
+		 * Check the current undo state.
+		 * @return {Boolean} Whether the document has future state to restore.
+		 */
+		undoable : function()
+		{
+			return this.enabled && this.hasUndo;
+		},
+
+		/**
+		 * Perform undo on current index.
+		 */
+		undo : function()
+		{
+			if ( this.undoable() )
+			{
+				this.save( true );
+
+				var image = this.getNextImage( true );
+				if ( image )
+					return this.restoreImage( image ), true;
+			}
+
+			return false;
+		},
+
+		/**
+		 * Perform redo on current index.
+		 */
+		redo : function()
+		{
+			if ( this.redoable() )
+			{
+				// Try to save. If no changes have been made, the redo stack
+				// will not change, so it will still be redoable.
+				this.save( true );
+
+				// If instead we had changes, we can't redo anymore.
+				if ( this.redoable() )
+				{
+					var image = this.getNextImage( false );
+					if ( image )
+						return this.restoreImage( image ), true;
+				}
+			}
+
+			return false;
+		}
+	};
+})();
+
+CKEDITOR.config.undoStackSize = 20;
Index: /CKEditor/trunk/_source/skins/default/toolbar.css
===================================================================
--- /CKEditor/trunk/_source/skins/default/toolbar.css	(revision 3085)
+++ /CKEditor/trunk/_source/skins/default/toolbar.css	(revision 3086)
@@ -331,5 +331,12 @@
 	background-position: 0 -880px;
 }
-
+.cke_skin_default a.cke_button_undo .cke_icon
+{
+	background-position: 0 -208px;
+}
+.cke_skin_default a.cke_button_redo .cke_icon
+{
+	background-position: 0 -224px;
+}
 .cke_skin_default a.cke_button_numberedlist .cke_icon
 {