"The professional schools will resume their professional responsibilities just to the degree that they can discover a science of design, a body of intellectually tough, partly formalizable, partly empirical teachable doctrine about the design process. " [H.A. Simon, 1968} Design is aimed at the transformation or translation of a specification or high­ level description into a description in terms of some real-world primitives. As such it involves the removal of the uncertainty about the way in which a required system can be realized. To optimally support the design of systems, we must look at the design process as a whole and at the strong relationship that exists between a designer, the applied design method, the required design tools and the ways in which designs can be expressed. This book focuses on that relationship. The application field we are concerned with is the design of systems in which the communication between system elements is a major design feature. Examples of such communicating systems are: communication protocols, telephone exchange control systems, process control systems, highly modular systems, embedded software, interactive systems, and VLSI systems. In summary, we are concerned with systems in which concurrency plays a major role (concurrency defines the mutual relationship between the activities in the different parts of a system or within a collection of systems).