A probabilistic approach to three-dimensional limit equilibrium slope stability is presented. Failure events involving long or very short failure zones are shown to be highly improbable. The existence of a fairly distinct most probable failure length is revealed and a method to determine it is proposed. It is further found that the mean margin of safety is twice as large as that predicted by plane strain analysis. The paper also calculates the risk that a failure will occur anywhere along an embankment of given total length, and identifies as the dominant contribution to the risk a term that is linear with embankment length. This result has major implications with respect to the safe design of long linear geotechnical facilities such as dams, levees, and highways. The proposed analysis accounts for other sources of uncertainty in slope stability calculations. It is an aid in design and exploration in the pursuit of the benefits of risk reduction.