This paper will discuss the design of controllers for systems which must interact dynamically with their environment. The conventional regulator design approach is unsuited to this task. Because the true effect of a controller on physical hardware is to modify its behavior, an alternative perspective is to describe the controlled hardware as an equivalent physical system. This permits the application of structured physical system modelling techniques to control system design. The application of this method to a robot performing contact tasks will be presented. It will be shown that the approach yields a successful design and leads to some useful insights.