“What Is a Petri Net?” Informal Answers for the Informed Reader -- Application Oriented Approaches -- The ?Petri Net Baukasten?: An Overview -- Improving the Usability of Petri Nets with the ?Petri Net Baukasten? -- Implementation of Parameterized Net Classes with the Petri Net Kernel of the ?Petrinetz-Baukasten? -- Process Landscaping: Modelling Distributed Processes and Proving Properties of Distributed Process Models -- Unifying Frameworks -- Petri Nets over Partial Algebra -- Parameterized Net Classes: A Uniform Approach to Petri Net Classes -- Behavior and Realization Construction for Petri Nets Based on Free Monoid and Power Set Graphs -- Rewriting Logic as a Unifying Framework for Petri Nets -- Theoretical Approaches -- Generalized Automata and Their Net Representations -- On Concurrent Realization of Reactive Systems and Their Morphisms -- Transactions and Zero-Safe Nets -- Two Algebraic Process Semantics for Contextual Nets -- Continuous Petri Nets and Transition Systems.

Since their introduction nearly 40 years ago, research on Petri nets has diverged in many different directions. Various classes of Petri net, motivated either by theory or applications, with its own specific features and methods of analysis, have been proposed and studies in depth. These successful developments have led to a very heterogeneous landscape of diverse models, and this, in turn, has stimulated research on concepts and approaches that contribute to unifying and structuring the diverse landscape. This state-of-the-art survey presents the most relevant approaches to unifying Petri nets in a systematic and coherent way. The 14 chapters written by leading researchers are organized in topical sections on application-oriented approaches, unifying frameworks, and theoretical approaches.