The objective of the project on Optimization of Nuclear Power Plant Overall Performance within the IAEA's subprogramme of Nuclear Power Planning, Implementation and Performance is to systematically improve the overall performance and compeititiveness of nuclear power plants (NPPs) with due regard to safety through the application of technological and engineering best practices, including quality assurance/quality management, and the utilization of relevant databases. As an integrated part of this project, the Technical Working Group on Life Management of NPPs deals with the managerial and engineering aspects of NPP maintenance, its optimization process with special regard to the importance of condition monitoring in maintenance strategies and the contribution of maintenance to managing the lifetime of operating NPPs. This publication was developed in the above framework with the objective to collect and analyse proven maintenance optimization methods and techniques (engineering and organizational) in Member States.