Petri nets are a well-known model for parallel systems, used for both applications and theoretical studies. They can be used for specification, modelling, and analysis, and offer a graphical representation and a clear view of concurrency. For the design of large systems, modular construction is indispensible, and considerable effort has been spent on studying themodular construction of Petri nets. This book studies the modular construction of nets, and in particular the top-down design of nets by action refinement. Suitable behavior descriptions are presented and special care is taken to justify these descriptions by showing that theyare necessary under reasonable specification requirements. In particular, itis shown that partial-order semantics is necessary to support action refinement.