The mechanism of slope failures has been the subject of extensive discussion and study in recent years. The role of progressive failure is now considered to be an important one in many situations. Failures which appear to occur quite suddenly are, in fact, preceded by the gradual formation of slip surfaces or shear bands. Deformation is often concentrated along these surfaces, and as a consequence of the strain-softening behavior of slope materials, the strength falls from peak to lower values. The minimum or limiting shear strength is the residual strength, and this may be operative over part or the whole of the whole of the shear band at any given stage of its development. Concepts from crack mechanics based on energy considerations have been used to study the conditions for the propagation of shear bands. Planar shear bands entirely at the residual strength paralled to the surface of a slope with a cut have been considered. The energy for driving the shear band is provided by the release of stresses due to the cut as well as by the difference in shear stress and shear strength over the length of the shear band. In this paper, two extensions to this work are considered. The first concerns the variation of strength from peak to the residual along the length of the shear band. This variation may be quite arbitary. It is shown that the proportion of the fall from the peak strength has a considerable influence on the criterion for failure propagation. Consequently, the manner in which strength actually varies along shear bands needs to be studied. The second extension concerns shear bands to arbitrary inclination within natural slopes. The energy approach is uded again but with one important difference. There is no cut in the slope and thus no release of stress. The energy for driving the shear band is provided entirely by the difference between the shear stress and residual shear strength along the shear band. This extension is a necessary step toward consideration of slip surfaces of arbitary shape within natural or altered slopes.