Focus is a framework for the systematic formal specification and development of distributed interactive systems and their components. Focus provides models, formalisms and verification calculi for the stepwise specification and development, transformation and verification of such systems. Focus aims at the modular development and implementation of distributed interactive systems through several abstraction levels by stepwise refinement. 1 Chapter 1 Methods for System Development A (distributed) system consists of a family of interacting, conceptually or spatially distributed components. A system development method provides a framework for organizing the stepwise construction of such systems. During the development process several descriptions are produced, that reflect different abstraction levels. Only if formal techniques are used these descriptions can be made as precise and unambiguous as necessary. Moreover, formal techniques allow to establish formal relationships between des...