Our first view of a concurrent process is that of a machine where every detail of its behaviour is explicit. We could take as our machine model automata in the sense of classical automata theory [RS59], also known as transition systems [Kel76]. Automata are fine except that they cannot represent situations where parts of a machine work independently or concurrently. Since we are after such a representation, we use Petri nets [Pet62, Rei85] instead. This choice is motivated by the following advantages of nets:

1. Concepts. Petri nets are based on a simple extension of the concepts of state and transition known from automata. The extension is that in nets both states and transitions are distributed over several places. This allows an explicit distinction between concurrency and sequentiality.

2. Graphics. Petri nets have a graphical representation that visualises the different basic concepts about processes like sequentiality, choice, concurrency and synchronisation.

3. Size. Since Petri nets allow cycles, a large class of processes can be represented by finite nets. Also, as a consequence of (1), parallel composition will be additive in size rather than multiplicative.

An attractive alternative to Petri nets are event structures introduced in [NPW81] and further developed by Winskel [Win80, Win87]. Event structures are more abstract than nets because they do not record states, only events, i.e. the occurences of transitions. But in order to forget about states, event structures must not contain cycles. This yields infinite event structures even in cases where finite (but cyclic) nets suffice.