ABSTRA CT A major aspect of user interface specification is concerned with describing the structure of dialogue between an interactive system and its users. The goal of this work is to establish an effective and usable language and process by which the behaviour of a user interface can be completely and accurately defined. This method should enable the production and maintenance of well-structured, understandable, and concise specifications for a wide variety of systems. It should also support specification at multiple levels of abstraction and allow refinement between these levels.