(1) Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, UK

(2) CNR - Istituto CNUCE, Pisa, Italy

Our aim is to support the design of interactive systems with important real-time and continuous aspects. This includes both the characteristics and capabilities of the systems themselves, and also the usability aspects. We present an approach to modeling which helps address these concerns in the early stages of design. In particular, we take into account the manipulation of objects or control of continuous processes in the environment via an interactive system. We discuss our initial approach with respect to some specific applications and properties, and refine the approach so that representations of the composite system within the entities comprising the system are taken into account. The message of this paper is that design oriented properties should be formulated with reference to a model (and understanding) of the relationships between user, system and environment.