This book presents the results of a probabilistic modeling of plane and wedge types of slope failures based on the Advanced First Order Second Moment (AFOSM) reliability method. For the investigation of both failure types, two different failure criteria are utilized. Then, due to the iterative nature of the AFOSM method, analyzing spreadsheets have been developed in order to carry out the computations. The developed probabilistic models and their spreadsheets are verified by investigating the affect of rock and slope parameters as well as their distribution types on the reliability index ( ), and probability of slope failure (PF). Moreover, Different probability distributions are used and the inverse transformation formulas of their non-normal variates to their equivalent normal ones are developed as well in this book. In addition, the probability of wedge failure is modeled by system reliability approach considering all the possible modes of failure in that case. Then, the failure probability of each mode is compared with the result of the system reliability approach.