The paper investigates possible interface waves between two solids in presence of separation and frictional slip and examines the effect of the waves on the sliding motion of the bodies. It is shown that interface waves are mathematically feasible between solids with identical mechanical properties, and that they can propagate with any phase velocity that falls between the Rayleigh and shear wave speeds. In the presence of such interface waves, global sliding may take place at much lower applied shearing tractions than would be required to slide the solids as rigid bodies. The waves also involve singular interface tractions that lead to energy emission and absorption at the singular points, and some aspects of them appear controversial.