A micromechanical model for friction of brittle elastic solids is introduced and applied to rock friction. It is assumed that contacts are characterized by an adhesive shear strength, a material property that is the only free variable in the formation. The net contact state of rough surfaces is calculated from elastic contact theory based on the measured topography of the surfaces. The model is tested by comparing it with friction measured with the same surfaces. Good agreement between theory and experiment was obtained from the initial application of shear stress to the onset of steady-state friction. The early stages of friction were predicted by the model based on the initial surface topographies and including the contributions of both normal and oblique contacts. Slip hardening was successfully modeled by including the evolution of the surface topography and contact state due to wear. ¿ American Geophysical Union 1995