The design and construction of interactive systems with high usability requires a user-centered approach to system development. In order to support such an approach, it is necessary to provide tools and representations reflecting a behavioral view of the interface—a view centered on user activities and the system activities and states perceived by the user. While behavioral representations exist, there is no behavioral model of interaction between a user and a system. Such a model is necessary for formalization and extension of existing behavioral representations.

This dissertation presents a model of interactive behavior based on the timetree, a novel tree-based structure representing tasks, user actions, system activity, and system and interface state, all within a framework of branching sequential timelines. The model supports formal definitions, operations and abstraction techniques. Three application areas—a formal definition of an existing behavioral notation, connection between a behavioral representation and a formal model of input devices, and techniques for analysis of behavioral specifications—provide examples of the model's utility.