Defence in depth has become a fundamental aspect of the analysis of the adequacy of technical systems to assure nuclear power plant safety. It is a comprehensive approach to providing a systematic means to analyse and assure layers of systems to prevent or mitigate accidents. This publication is intended to provide a philosophy to guide the thinking about the institutional structures necessary to assure nuclear safety. It refers to the three important institutional subsystems – the industry, regulator and stakeholders – and describes the interfaces that should be nurtured among these as well as within each subsystem. The publication is intended to serve as a fundamental tool in the continuing efforts to strengthen nuclear safety.