The initial Layer of protection analysis LOPA book published in 2001 set the rules and approaches for using LOPA as an intermediate method between purely qualitative hazards evaluationanalysis and more quantitative analysis methods. Basic LOPA provides an order-of-magnitude risk estimate of risk with fairly reproducible results. LOPA results are considered critical in determining safety integrity level for design of safety instrumented systems. pThis guideline clarifies key concepts and reinforces the limitations and the requirements of LOPA. The main scope of the guideline is to provide examples of CMs and ECs and to provide concrete guidance on the protocols that must be followed to use these concepts. The book presents a brief overview of Layer of Protection Analysis LOPA and its variations, and summarizes terminology used for evaluating scenarios in the context of a typical incident sequence. It defines and illustrates the most common types of ECs and CMs and shows how they interre