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 behavior of critical systems. This thesis uses the theory of Communicating Sequential Processes to show how a real-time system (a system that maintains a continuous interaction with its environment) 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 for structuring CSP descriptions of layered protocols is given. The research contained here represents the very latest work on the specification and verification of real-time systems.