Petri nets are a well-known model for parallel systems, usedfor both applications and theoretical studies. They can beused for specification, modelling, and analysis, and offer agraphical representation and a clear view of concurrency.For the design of large systems, modular construction isindispensible, and considerable effort has been spent onstudying themodular construction of Petri nets.This book studies the modular construction of nets, and inparticular the top-down design of nets by action refinement.Suitable behavior descriptions are presented and specialcare is taken to justify these descriptions by showing thattheyare necessary under reasonable specificationrequirements. In particular, itis shown that partial-ordersemantics is necessary to support action refinement.