This publication provides specific technical information on implementation of the defence in depth concept in the siting, design, construction and operation of nuclear power plants (NPPs). It describes a method for verifying capabilities for implementation of defence in depth in existing NPPs. For given objectives of each level of defence, a set of challenges to achieve these objectives is identified as well as several constitutive mechanisms leading to these challenges, and a list of possible safety provisions which contribute to prevention of these mechanisms is provided. This book is intended to serve as a reference primarily for self-assessment of the comprehensiveness and quality of defence in depth provisions by NPP operators, but it can also be used by regulators or independent reviewers. It offers a complementary tool for evaluation of the strengths and weaknesses of defence in depth in a specific NPP.