The main aims of the series of volumes "Advances in Petri Nets" are: - to present to the "outside" scientific community a fair picture of recent advances in the area of Petri nets, and - to encourage those interested in the applications and the theory of concurrent systems to take a closer look at Petri nets and then join the group of researchers working in this fascinating and challenging area. This volume is based on the proceedings of the 12th International Conference on Applications and Theory of Petri Nets, held in Gjern, Denmark, in June 1991. It contains 18 selected and revised papers covering all aspects of recent Petri net research.