This thesis examines the influence of the variation of joint orientations on the stability of rock slopes. A landslide in a quarry in 2007 allows a back analysis of strength parameters. The affected slope consists of faulted and jointed rock. The local geological structures as well as significant structures on the slopes surface discovered after the failure indicate a toppling failure. Analytical and numerical calculations are performed. According to ÖNORM B 1997-1-5, stability analyses of rock slopes are to be carried out, taking into account a variation of joint orientations. Further, structural safety is proved by using a partial safety concept. In this study, the uncertainties of the joint orientations are statistically analysed and represented by distribution functions. The analytical calculations are performed using the software RocTopple which is based on the limit equilibrium method for toppling failure by Goodman & Bray (1976). RocTopple offers probabilistic features to consider distribution functions for any parameter. Accordingly, Monte-Carlo-simulations are used for the analytical calculations. For numerical calculations, done with the software UDEC, the variations of the joint orientations must be considered and modelled individually. Based on the distribution function, for further calculations a relatively small number of orientations is chosen. This approach is similar to the "Point Estimate Method". After modelling a restoration geometry and the geological conditions, the stability for different combinations of the joint orientations is determined. The results show that the variations of the joint orientations have more influence on the overall stability of the quarry wall than the variation of the strength parameters. The results of this study also indicate considering the uncertainties of joint orientations with probabilistic methods for safety calculations in rock mechanics.