Nuclear power reactors have become a major source of electricity supply in many countries and, based on the experience of safe and reliable operation, many operators have sought and received authorisation for long-term operation beyond the period assumed in the plant’s design. Acceptance of a nuclear power plant for long-term operation must be based on evidence that the plant will operate safely over the extended period of service. This requires an assessment of the current and projected condition of the plant and, in particular, of the systems that perform fundamental safety functions, to ensure that these systems will continue to perform their safety functions during the extended operating period. Programmes for long-term operation must be informed by operating experience and must also consider and assess environmental impacts. This guidance document is intended to assist regulatory organisations in assessing and approving the long-term operation safety assessments submitted by operators. It outlines the fundamental principles that should govern decisions on authorisation for long-term operation. It also describes regulatory challenges and considerations that may arise in an assessment of a plant for long-term operation.