The importance of formal methods in software engineering has been receiving greater acknowledgement. These methods can be used at several stages of the software development process. This book focuses on a method concerning the early stages of design, namely the modeling of a system at conceptual level, and the verification and validation of this model.The mathematical formalism used for modeling and verifying systems is the synchronized product of transition systems. The book introduces this notion and presents several examples of modeling and verification covering various domains: games, industrial processes, communication protocols, etc.These examples are worked out using the "Mec" software tool. This book is also intended to be an introduction to this tool which is described in full detail.