The principal methods of core design uncertainly analysis are critically reviewed. The overconservatism of the Deterministic Method, which aims at ensuring that the design limits cannot be exceeded in the most loaded channel (or at the most loaded spot), leads to a probabilistic approach (the Statistical Method) in which the probability of such an event is evaluated. Recent work in this direction is discussed. It is emphasized, however, that a probabilistic reliability evaluation must cover the whole core, and not only its most heavily loaded element. The Synthesis Method presented here fulfills this requirement without demanding the use of computers. The Synthesis Method also allows the use of a realistic space-dependent reliability criterion. The various methods under review are compared in their application to a fast gas-cooled reactor core. The power levels corresponding to a given reliability are calculated and the Synthesis Method is seen to be more conservative than the classical Statistical Method and less conservative than the Deterministic Method.