Traditionally Vaadin widgets use simple paint-change cycle to synchronize the client and server parts of a UI component. Widget RPC introduces a new way of communicating using method calls in more RPC-like manner and enables developers to build more fine-grained communication more easily. First, register a listeners: rpc.register("doStuffWithThings", new Method() { void invoke(String methodName, Object[] params) // do stuff with things } }); Then you can invoke methods like this: rpc.call("doStuffWithThings", aThing, anotherThing); This works both ways (client to server and server to client). See the highlights and documentation for details. .... Even this mechanism is in use in several of my components like the Console, SWFObject and YouTubePlayer, I consider it as "experimental" until I get some positive feedback and the users are comfortable with the API. NOTE: For Vaadin 7 has its own very similar mechanism and this component is not needed.