Abstract Traditionally, both deterministic and probabilistic methods have been used in the safety analysis of nuclear reactors. Both of these methods have evolved in recent years to a more sophisticated treatment of uncertainties in the anallyis. In particular, deterministic analysts have moved away from the traditional conservative methods to best-estimate techniques that attempt to account for uncertainties in all phases of the calculations. Because of this, a tighter and more formal coupling between the deterministic and probabilistic methods has become possible. This paper introduces a formal methodology for such combination, presents an illustrative example, and discusses a new technique for deriving acceptance criteria for deterministic analyses based on probabilistic considerations.