This paper focuses on the importance of three-dimensional (3D) slope stability analyses in practice. Commercially available 3D slope stability software does not consider the shear resistance along the two sides of the slide mass that parallel the direction of movement in calculating the 3D factor of safety (Stark and Eid 1998). Consequently, the 3D factor of safety may be underestimated and the back-calculated shear strengths may be overestimated or unconservative. A method for incorporating the shear resistance along the two sides of a slide mass in existing 3D software is presented. A parametric study is used to investigate the importance of 3D end effects by providing a comparison of two-dimensional (2D) and 3D analyses for various slide mass geometries and shear strengths along the failure surface. A field case history is used to illustrate the use of the parametric study results and the importance of conducting a 3D analysis in practice.