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, where 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 can be understood as a method for specifying any given system or any section of such a system to any given degree of refinement. The simple and immediately understandable principles of system modelling with nets makes it possible to provide an illustrated description of this method without going into the mathematics behind it. The text is based on courses the author developed for project engineers and project managers in the area of embedded computer systems.