[en] The number of nuclear facilities being or planned to be shutdown as they reach the end of their design life, due to accidents or other political and social factors has been increasing worldwide. This has led to an increase in the awareness of regulators and operators of the importance of development and implementation of adequate safety requirements and criteria for decommissioning of these facilities. A general requirement at international and national levels, even for new facilities to be commissioned, is the development of a decommissioning plan, which includes evaluation of potential radiological consequences to public and workers during planned and accidental decommissioning activities. Experience has been gained in the safety assessment of decommissioning at various sites with different complexities and hazard potentials. This experience shows that various approaches have been used in conducting safety assessments and that there is a need for harmonisation of these approaches and for transferring the good practice and lessons learned to other countries, in particular developing countries with limited financial and human resources. The IAEA launched an international project on Evaluation and Demonstration of Safety during Decommissioning (DeSa) in 2004 to provide a forum for exchange of lessons learned between site operators, regulators, safety assessors and other specialists in safety assessment of decommissioning of nuclear power plants, research reactors, laboratories, nuclear fuel cycle facilities, etc. This paper presents the lessons learned through the project up to date, i.e.; (i) a common approach to safety assessment is being applied worldwide with the following steps - establishment of assessment framework; description of the facility; definition of decommissioning activities; hazard identification and analysis; calculation of consequences; and analysis of results; (ii) a deterministic approach to safety assessment is most commonly applied; (iii) a comprehensive approach to identification of radiological and non-radiological hazards needs to be applied, as in most decommissioning projects non-radiological hazards are predominant; (iv) there is limited experience worldwide on procedures for review of decommissioning safety assessments; (v) a graded approach is being applied on the basis of various criteria and there is no internationally agreed common approach; (vi) engineering evaluation of systems and components remains a challenge due to the gradual elimination of engineering barriers, aging of a facility and dynamic decommissioning activities; (vii) more clarity and guidance is needed on the interface between safety assessment for waste management and decommissioning. It is envisaged that consolidation of the knowledge, experience and lessons learned from decommissioning in over thirty Member States will provide a useful tool for the successful performance and regulation of decommissioning and for the release of sites from regulatory control consistent with international safety standards and good international practice. (author)