Spatial variability and model uncertainty are considered the major sources of geotechnical uncertainty. The primary objective of this paper is to provide slope stability investigators with a robust reliability framework that takes into consideration the combined uncertainty of spatial variability and model uncertainty. To achieve this objective, a quantification of the model uncertainty of common slope stability models is conducted by assembling and analyzing a database of historical failures of slopes. The database is also used to investigate the possibility of a lower-bound factor of safety and its impact on the reliability of slopes. It is concluded that both spatial variability and model uncertainty in the undrained shear strength of clays have a direct effect on the mean of the factor of safety and its coefficient of variation. Moreover, it is found that the lower-bound factor of safety can cause a significant increase in the calculated reliability for an undrained slope