This book commemorates the work done by Tony Hoare and published under the title Communicating Sequential Processes in the 1978 August issue of the Communications of ACM. The British Computer Society's specialist group Formal Aspects of Computing Science organized a meeting on July 7-8, 2004, in London, to mark the occasion of 25 years of CSP. The 19 carefully reviewed and revised full papers by leading researchers celebrate, reflect upon, and look beyond the first quarter-century of CSP's contributions to computer science. The papers explore the impact of CSP on many areas ranging from semantics and logic, through the design of parallel programming languages to applications varying from distributed software and parallel computing to information security, Web services, and concurrent hardware circuits.