Slope reliability must be considered in the context of a variety of factors which control or influence stability or the balance between disturbing and resisting forces. The problems of slope stability are varied, ranging from fi rst-time failures to reactivated landslides and from minor, localized slope movements to catastrophic events. Reliability analysis within a probabilistic framework offers a very powerful tool for taking into consideration the variability of key geotechnical parameters as well as other uncertainties. In this paper, the progressive decrease in shear strength along potential slip surfaces is considered in terms of a residual factor treated as one among several random variables in the slope reliability formulation. Attention is restricted to natural slopes in which the use of an infinite slope model is appropriate and in which pore water pressure as well as the residual factor are very important. However, the proposed reliability approach can be extended to complex slopes with curved slip surfaces of arbitrary shape.