In any design process, there are different perspectives that need to be accommodated. For the design of interactive systems, two of these are that of the computer system designer and that of the end user. The focus of this paper is on tools or notations to support the integration of these different perspectives-in particular, system modelling and theorybased user modelling. There are few established techniques for doing such integration; those that there are generally involve a loose coupling between the description of the system and that of the user, or are skewed towards one or other of these viewpoints. Recently, techniques that deal with the system and user more symmetrically have emerged. We focus, in particular, on techniques that have been developed and investigated within a large European project, Amodeus. These allow the analyst to explore properties of the conjoint system, and to investigate how the properties of the interaction relate to those of the individual agents. However, there is a trade-off, which can be characterized as increased power to critique a design being offset against reduced generality in the design perspectives that can be considered using the technique. We argue that while such techniques require a high initial investment, technological advances make the need for such integrated approaches urgent. We consider what resources each class of technique demands and what kinds of results each yields.