This volume contains the proceedings of the 20th International Conference on Application and Theory of Petri Nets. The aim of the Petri net conferences is to create a forum for the dissemination of the latest results in the application and theory of Petri nets. Typically there are some 150-200 participants and usually one third of these come from industry, while the rest are from universities and research institutions. The conferences and a number of other activitiesare co- dinatedbyasteering committeeformedby: G.Balbo (Italy),J. Billington(A- tralia), C. Girault (France), K. Jensen (Denmark), S. Kumagai (Japan), G. De Michelis (Italy), T. Murata (U.S.A.), C.A. Petri (Germany; honorary member) W. Reisig (Germany), G. Roucairol (France), G. Rozenberg (The Netherlands; chair), M. Silva (Spain). The 1999 Petri net conference took place in Williamsburg, Virginia, and was organized by the Department of Computer Science of The College of William and Mary, Williamsburg. This was the second time the conference had been organized in the United States. We received 45 submissions from 15 countries on 5 continents of which 21 accepted for presentation.The submitted papers were evaluated by a program committee with the following members: W. van der Aalst (The Netherlands), P. Azema (France), W. Brauer (Germany), S. Christensen (Denmark), A. Desrochers (U.S.A.), S. Donatelli (Italy; co-chair), C. Girault (France), L. Gomes (Portugal), J. Hillston (United Kingdom), E.
