The software control systems that are embedded in many products are increasingly complex. There are many stages in the typical software design life cycle, and these include testing, and sometimes use formal methods. This thesis aims to strengthen the design life cycle by demonstrating a pragmatic use of formal methods for an industrially-applicable design method. We take a design method and notation called DORIS (which is used by British Aerospace), and give a formal syntax for the notation. This then forms the structure into which we place activities (the active processing parts of the system), and intercommunication data areas (IDAs, the ‘passive’ parts of the system through which activities communicate). We aim to give an industrially useful semantics for this notation, so we use several models of Timed CSP as the underlying semantic domain. This allows us to represent the many timed and liveness requirements in BAe’s work using the well-understood theory for Timed CSP.