Petri nets can express concurrency and nondeterminism but not hierarchy. This article presents an extension of Petri nets, in which places can be grouped into so-called " units " expressing sequential components. Units can be recursively nested to reflect the hierarchical nature of complex systems. This model called NUPN (Nested-Unit Petri Nets) was originally developed for translating process calculi to Petri nets, but later found also useful beyond this setting. It allows significant savings in the memory representation of markings for both explicit-state and symbolic verification. Six tools already implement the NUPN model, which is also part of the next edition of the Model Checking Contest.