This book constitutes the proceedings of the 42nd International Conference on Application and Theory of Petri Nets and Concurrency, PETRI NETS 2021, which was held virtually in June 2021. The 22 full papers presented together with 2 keynote papers in this volume were carefully reviewed and selected from 39 submissions. The focus of the conference is on the following topics: application of concurrency to system design; games; verification; synthesis and mining; reachability and partial order; semantics; and tools.