This book constitutes the refereed proceedings of the 21st International Conference on Application and Theory of Petri Nets, ICATPN 2000, held in Aarhus, Denmark, in June 2000. The 20 revised full papers presented together with four invited surveys and four tool presentations were carefully reviewed and selected from 57 submissions. The papers address all current aspects of Petri net research and development including system design and verification, UML, compositionality, process algebras, model checking, computer networking, business process engineering, communication networks, etc. Various classes of Petri nets are discussed including safe Petri nets, high-level Petri nets, colored Petri nets, P/T nets, and timed Petri nets.