L¿ders' bands are shear deformation features commonly observed in rock specimens that have been deformed experimentally in the brittle-ductile transition regime. For specimens that contain both faults (shear fractures that separate the specimen) and bands the bands from earlier in the deformation history, and their orientations are often different from the fault. These differences pose the question of the relationship between these two structures. Understanding the origin of these features may shed light on the genesis of apparent natural analogues and on the general process of rock deformation and fracture in the laboratory. This paper presents a hypothesis for the formation of L¿ders' bands in laboratory specimens based on deformation localization theory considered in the context of the nonuniform stress distribution of the conventional triaxial experiment. L¿ders' bands and faults appear to be equivalent reflections of the localization process as it is controlled by nonuniform distributions of stress and evolution of incremental constitutive parameters resulting from increasing damage. To relate conditions for localization in laboratory specimens to natural settings, it will be necessary to design new experiments that create uniform stress and deformation fields or to extract constitutive data indirectly from standard experiments using computational means. ¿ 2000 American Geophysical Union