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 an analogous 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.