Analysis of the long-term safety of radioactive waste repositories, using performance assessment and other tools, is required prior to implementation. The initial stage in developing a repository safety assessment is the identification of all factors that may be relevant to the long-term safety of the repository and their combination to form scenarios. This must be done in a systematic and transparent way in order to assure the regulatory authorities that nothing important has been forgotten. This report is a review of developments in scenario methodologies based on a large body of practical experience in safety assessments. It will be of interest to radioactive waste management experts as well as to other specialists involved in the development of scenario methodologies.