There are two main approaches to a theory of concurrent distributed computations: the theory of Petri nets and the Milner/Hoare theory of CCS/CSP. They are based on different philosophies and emerged from two different classical notions of computability. The Petri net approach developed (in the early 60s) from the ideas around Turing machines and automata; it has concurrency and causality as its basic concepts. CCS/CSP grew (in the middle of the 70s) out of ideas around the A-calculus and concepts in programming; it has communication and composition as its basic notions.