The migration of a user interface (UI) is the action of transferring a UI from one device to another, for example from a desktop computer to a handheld device. A UI is said to be migratable if it has the ability to migrate. This paper describes how the QTk toolkit has been extended to provide a migratable UI and the application programming interface (API) provided to the developers. Basically, an indirection layer has been introduced between the application and the actual representation of the UI. The migration of a UI is achieved by firstly creating a clone of the state of the site displaying the UI, secondly by changing the indirection to point to this clone. The API provides a way to specify if (the entirety of) a window can be migrated or not at construction time. A migratable window returns a universal reference that can be given to any site with whom a network connection is possible. This reference can be used by a receiver widget to migrate the window there. Interestingly, a migratable window can itself contain a receiver widget configured to display the content of another migratable window: all windows are transparently migrated. Also a window (stationary or migratable) may contain one or more receiver widgets : it is possible to dynamically compose a UI from several different UIs.