Nuclear power plants (NPPs) are increasingly operating for longer time frames than were originally anticipated at their launch. Out of the total number of NPPs operating in the world, many have been in operation for more than 30 years, and half for more than 20 years. This publication fulfils, in part, the urgent needs expressed by Member States for information resources to aid the development of national programmes for safe long term operation of NPPs. The publication focuses on the physical systems, structures and components of NPPs.