The Probabilistic System Assessment Group (PSAG) was established by the RWMC in January 1985 (originally under a different name) to help co-ordinate the development in OECD member countries of probabilistic safety assessment computer codes to be used in performance assessment of radioactive waste disposal facilities. The PSAG met approximately twice per year between its founding and its final meeting in June 1994. A key part of the Group’s activities was the conduct of code intercomparison exercises aimed at building confidence in the correct operation of probabilistic assessment codes being prepared for applications in national programmes. Such exercises stimulated the thinking process and helped pave the way for basic developments, improvements and advancements in the application of probabilistic methods to assessment of waste disposal systems. In addition, the PSAG discussed many topical issues of relevance to PSA code development and to the whole question of the treatment of uncertainty in performance assessment. The PSAG reported to, and had its work reviewed by, the NEA Performance Assessment Advisory Group (PAAG).