The primary purpose of design requirement criteria for reactor safety systems is to better ensure the adequacy of safety system design, the single failure and separation criteria being cases in point. However, strict adherence to these two criteria, for example, may actually result in a less than optimal design in terms of system reliability. Working within the spirit of these criteria, an integration of probabilistic analyses into the licensing review process for safety system design would result in a more reliable system, which, after all, is the intent of deterministic criteria. Current probabilistic analysis techniques are adequate for even complex safety system designs. However, in applying specific probabilistic tools, the analyst must not only be familiar with their merits/limitations, but must also be knowledgeable of the system (and supporting hardware) under consideration.