This report was prepared in response to the recommendation of the IAEA General Conference in 1992 that the Director General develop a process to provide a common basis on which the acceptable level of safety of all operating nuclear power plants built to earlier standards can be judged. This followed a recommendation made at the IAEA Conference on 'The Safety of Nuclear Power: Strategy for the Future' held in September 1991. The report also provides a basis for allocating responsibilities for, and deciding on the best approach to, making a safety assessment, and for selecting criteria to adopt in deciding on what corrective action to take.