Petri nets provide a formal framework for system modeling and validation which has proven to be very reliable in practice. This book presents various net models appropriate for designing specific systems. Here, systems are understood very generally as "organizational systems" in which regulated flows of objects and information are significant. The models are interrelated in the sense that they have common interpretation patterns: together they provide a method for specifying any given system or any section of such a system to any given degree of refinement. The book begins with a chapter of examples that illustrate the essential principles of system design with nets. Then it moves systematically from special to higher net models and finally arrives at a set of guidelines for system design with nets. Various smaller examples, exercises with solutions, and an an extended example of a wholesale company in the last chapter provide practice in applying the method.