Disposal of high-level and/or long-lived radioactive waste in engineered facilities, or repositories, located underground in suitable geological formations, is being widely investigated world-wide as a long-term management solution. This is in order to protect humans and the environment, both now and in the future. From a quantitative point of view, a repository is said to be safe if it meets the relevant safety standards, such as internationally recommended or specified by the responsible national regulatory authorities. In recent years the scope of the safety assessment has broadened to include the collation of a broader range of evidence and arguments that complement and support the reliability of the results of quantitative analyses. The broader term “post-closure safety case”, or simply “safety case”,1 is used to refer to these studies. It has also become evident that repository development will involve a number of step-by-step stages, punctuated by interdependent decision making on whether and how to move from one stage to the subsequent one. These decisions require a clear and traceable presentation of robust technical arguments that will help to give confidence in the feasibility and safety of a proposed concept. The depth of understanding and technical information available to support decisions will increase from step to step. The safety case is a key input to support a decision to move to the next stage in repository development. It reflects the state of understanding and the results of the research and development (R&D) undertaken at a certain stage, and supports decisions concerning future R&D efforts.