The aim of this work is to provide a practical introduction to the formal specification of concurrent and real-time systems. It is intended for those with some basic knowledge or experience of computing but does not assume knowledge of the particular problems of concurrent and real-time systems. It concentrates on one group of languages, based on Robin Milner's Calculus of Communicating Systems (CCS). Supported throughout by examples and exercises, the practical applicability of the method is demonstrated and the benefits of a rigorous approach made clear. This text recognizes that building formal specifications, using abstraction and formal reasoning, are difficult skills to master, and thus adopts a practical, student-friendly approach.