This book constitutes the refereed proceedings of the 20th International Conference on Application and Theory of Petri Nets, ICATPN'99, held in Williamsburg, Virginia, USA, in June 1999. The 21 revised full papers presented were carefully selected from 45 submissions. Also included are three invited presentations. The book presents state-of-the-art research results on all current aspects of Petri nets as well as advanced applications in a variety of areas.