The stability assessment of earth slope is one of the fundamental problems in geotechnical engineering and may be addressed using both deterministic and probabilistic approaches. The deterministic analysis basically involves the calculation of safety factor for trial slip surfaces and search for a slip surface yielding the lowest factor of safety. In probabilistic approach, the safety of a slope is measured by the reliability index or probability of failure. The problem of searching the minimum factor of safety and minimum reliability index of earth slope can be formulated as an optimization problem. In this book, first deterministic analysis of earth slopes based on limit equilibrium method of slices will be discussed. The focus then shifts to the topic of probabilistic approach and reliability analysis of earth slope. Afterwards, available nature inspired optimization techniques will be reviewed and an effective global optimization algorithm based on gravitational search algorithm will be presented. Also, application of the new algorithm for stability assessment of earth slopes and search for the critical deterministic and probabilistic slip surfaces will be discussed in detail.