This book offers an overview of the key ideas of Petri nets, how they were developed, and how they were applied in diverse applications. The chapters in the first part offer individual perspectives on the impact of Petri’s work. The second part of the book contains personal memories from researchers who collaborated with him closely, in particular they recount his unique personality. The chapters in the third part offer more conventional treatments on various aspects of current Petri net research, and the fourth part examines the wide applications of Petri nets, and the relationships with other domains. The editors and authors are the leading researchers in this domain, and this book will be a valuable insight for researchers in computer science, particularly those engaged with concurrency and distributed systems.