Abstract : In addition to highlighting the benefits of the use of Petri Nets (PN) in system’s dependability studies, this book, which is divided into four parts, also shows the risk analysis step for probabilistic assessment of RAMS. In the first part, the basic models of Petri Nets and some useful extensions are briefly recalled. In the second part, Petri Nets are used as a formal model for describing the evolution process of a critical system in the framework of an ontological approach. The third part focuses on Stochastic Petri Nets (SPN) and their use in dependability assessment. Different formal models of Stochastic Petri Nets are formally presented (semantics, evolution rules, etc.) in addition to their equivalence with the corresponding class of Markov processes in order to obtain an analytical assessment of dependability. Simplification methods are proposed so as to reduce the size of the analytical model and thus make it more calculable. The introduction of some concepts specific to high-level Petri Nets also allows the consideration of complex systems. In the fourth part, five examples of industrial systems modeling are presented; they are issued from collaborations with large industrial companies in various application areas.