This book was first published in 1993. Computing systems are becoming highly complex, harder to understand, and therefore more prone to failure. Where such systems control aircraft for example, system failure could have disastrous consequences. It is important therefore that we are able to employ mathematical techniques to specify the behaviour or safety critical systems. This thesis uses the theory of Communicating Sequential Processes (CSP) to show how a real-lime system may be specified. Included is a case study in which a local area network protocol is described at two levels of abstraction, and a general method 14 structuring CSP descriptions of layered protocols is given.