Abstract We present a theory which describes enhanced Raman scattering from molecules adsorbed on metals. The enhancement is due to the screening fields induced by the optically polarized adsorbate in the presence of the substrate. These fields interact coherently with the applied optical field and the vibrational motion of the adsorbed molecule. The enhancement is shown to be coverage dependent and also rather sensitive to the optical dielectric function of the substrate. Predicted enhancements compare well with experiment.