The accident at the Fukushima Daiichi Nuclear Power Plant underlined the need to assess the nuclear safety of multi-unit sites considering the accident sequences involving more than one reactor units on site. The objective of this Safety Report is to provide a methodology for the development of a Multi-unit Probabilistic Safety Assessment (MUPSA). It provides practical examples and an overview of the actual state of practice in this area. The publication provides a detailed description of Level 1 MUPSA methodology, the principles of development of Level 2 MUPSA models and the path forward for multi-unit consequence analysis (Level 3 MUPSA). In addition, it summarizes the experience available in Member States in the area of MUPSA. The scope of this Safety Report includes consideration of various hazards and plant operational states normally considered in PSA development in the multi-unit context.