Book excerpt: This book constitutes the refereed proceedings of the 26th International Conference on Applications and Theory of Petri Nets, ICATPN 2005, held in Miami, USA in June 2005. The 20 revised full regular papers and 3 revised tool presentation papers presented together with 4 invited papers were carefully reviewed and selected from 69 submissions. All current issues on research and development in the area of Petri nets are addressed, in particular concurrent systems design and analysis, modular systems development, formal specification, model validation, model checking, workflow management, flow charts, networking, formal methods in software engineering, etc.