This publication addresses relevant aspects of requesting and obtaining effective technical support (TS) and its adequate utilization in decision making on nuclear power programmes, projects and plants. It describes the TS functions and associated organizational activities and skills in providing technical and scientific input to the decisions on plant safety and performance throughout the plant's lifecycle and serves as a guidance for establishing and sustaining TS capability and capacity in Member States both embarking on nuclear power programmes and operating nuclear power plants. The publication also presents observations, lessons learned and conclusions drawn from good practices for defining and maintaining roles, responsibilities and interfacing requirements of technical support organizations (TSOs), nuclear power project/plant entities and other stakeholders. As such, it provides a set of descriptive and practiced processes that integrate technical and scientific information for safety, performance and economical aspects in support of sound and timely decisions on the safe, reliable and efficient operation of nuclear power plants.