This book describes the design phase of the SIS safety life cycle as defined in IEC 61511:2016. Starting with a description of the entire safety life cycle process, the authors show how the design steps fit into that process starting with conceptual design through design verification. The book explains the advantages of the performance-based approach to design and provides the theoretical background for the probabilistic calculations that are the foundation of performance verification. The book also explains minimum redundancy concepts and equipment qualification. By providing numerous examples, the authors explain potentially confusing language from IEC 61511 and IEC 61508. Appendices include statistics, probability, failure-rate data tables, and system architectures. Each chapter contains questions and answers similar to those found on professional certification exams for functional safety, which makes the book a valuable resource for those seeking to achieve personnel certification.