4.3. The development of a geological disposal facility involves an iterative process of site characterization and the design and evolution of the safety case and the supporting safety assessment to provide an optimized level of operational and post-closure safety (see the appendix of Ref. [1]). Geological disposal facilities for radioactive waste may be developed and operated over a period of several years or several decades. Key decisions, such as decisions on the choice of concept, site selection and evaluation, design, construction, operation and closure of the disposal facility, are expected to be made as the project develops. In this process, decisions are made on the basis of information available at the time, which may be qualitative and/or quantitative, and on the confidence that can be placed in that information. Facility development, operation and closure decisions are influenced by external factors, such as national policy and preferences. "An adequate level of confidence in the safety of each disposal facility has to be developed before decisions are taken" [1].