This handbook documents process algebra (a formal description technique for complex computer systems) from its modern inception in the late 1970s to the present. Bergstra and A. Ponse (both of the U. of Amsterdam programming research group) and S.A. Smoka (computer science, State U. of New York present 19 contributions that discuss basic theory, finite-state and infinite-state processes, extensions to process algebra that make it easier to model the kinds of systems that arise in practice, non-interleaving process algebras, and tools and applications.