Communicating Sequential Processes (CSP) is an abstract language for describing processes and reasoning about their interactions within concurrent systems. It is appropriate for investigating the overall behaviour that emerges. It has a mature theory, and powerful tool support [For03] and can be considered as an off-the-shelf framework which can be customised for particular domains through additional domain-specific constructions and theory. This chapter introduces the application of CSP to the analysis of security protocols. For the purposes of this chapter, we will introduce only those parts of CSP that we use in protocol analysis. Fuller descriptions of the language and theory can be found in [Hoa85,Ros97,Sch99].