In underground repositories for radioactive waste, significant quantities of gases may be generated as a result of several processes. The potential impact of gas generation, accumulation and migration on the performances of the various barriers and, ultimately, on the long-term safety of a repository, should therefore be assessed in the development of safety cases for underground repositories. It was in this context that the EC and the NEA organised a workshop on "Gas Generation, Accumulation and Migration in Underground Repository Systems for Radioactive Waste: Safety-relevant Issues" in Reims, France on 26-28 June 2000. This book includes the texts of the invited presentations, the reports of the deliberations held in the five working groups, as well as the main conclusions of the workshop.