All national radioactive waste management authorities recognise today that a robust safety case is essential in developing disposal facilities for radioactive waste. To improve the robustness of the safety case for the development of a deep geological repository, a wide variety of activities have been carried out by national programmes and international organisations over the past years. The Nuclear Energy Agency, since first introducing the modern concept of the ?safety case?, has continued to monitor major developments in safety case activities at the international level. This Sourcebook summarises the activities being undertaken by the Nuclear Energy Agency, the European Commission and the International Atomic Energy Agency concerning the safety case for the operational and post-closure phases of geological repositories for radioactive waste that ranges from low-level to high-level waste and for spent fuel. In doing so, it highlights important differences in focus among the three organisations.