The safety assessment of geotechnical projects is traditionally carried out in a deterministic way, where the material properties, problem geometry and external forces are known fixed values. However, it is known that such parameters is affected by uncertainties and, for this reason, it is current the adoption of safety factors, characterized as the ratio between the material resistance and the instability actions. Since the safety factor is subject to parameter variability and sampling errors, this should not be taken as the only form of safety assessment. Hence, probabilistic methods have been used in the evaluation of slope instability risk, where the parameters are represented by the probability function. Probabilistic stability studies lead to results presented in the form of probability of failure. This work presents an evaluation of the probabilistic methods most used in estimating the probability of slope failure, with application to the case of the Santa Branca Dam.