Probabilistic Safety Assessment (PSA) is a structured, comprehensive, and logical analysis method aimed at identifying and assessing risks in complex technological systems, such as the nuclear power plants. It is also known as probabilistic risk assessment - PRA. This book presents the theoretical basis to understand the numerous and complex aspects that are covered by PSA and it will help the reader to better understand and to effectively manage risks. The book provides PSA methods and techniques and it includes recommended procedures that are based on the experience of the authors and applicable to different levels and types of PSA that are used for nuclear power plants applications. It can be used as extra reading for PSA courses for practitioners and it provides quantitative risk methodology documentation for PSA.