The main idea behind the series of volumes Advances in Petri Nets is to present to the general computer science community recent results which are the most representative and significant for the development of the area. The papers for the volumes are drawn mainly from the annual International Conferences on Applications and Theory of Petri Nets. Selected papers from the latest conference are independently refereed, and revised and extended as necessary. Some further papers submitted directly to the editor are included. Advances in Petri Nets 1990 covers the Tenth International Conference on Applications and Theory of Petri Nets held in Bonn, Germany, in June 1989. Additional highlights of this volume include a tutorial on refinements of Petri nets by W. Brauer, R. Gold, and W. Vogler, and a tutorial on analysis and synthesis of free choice systems by J. Esparza and M. Silva, both prepared in the framework of the ESPRIT Basic Research Actions Project DEMON.