The aim of this chapter is to introduce the general concepts of communication protocols and the terminology used in the field. The notions of a communication service, protocol, service access point, primitives and protocol messages are first explained. Then the general constituents of protocol messages are described. The implementation of protocols with protocol entities and channels is discussed, as well as the interaction of protocol entities with users. The model of a simple communication system is shown. A more detailed model of a protocol entity is then presented. A simple communication system is formally specified in SDL, using both a hierarchical and a one-level specification technique, and the methodological difference between the two presentations is emphasised. At the end of the chapter, the transfer of a user message through a simple communication system, consisting of two users, two protocol entities and a channel, is specified in SDL, both to offer a simple example of SDL specification and to demonstrate how user messages are transferred through a communication system. This chapter has 18 figures.