This review treats the current state of studies on the optical properties of metal surfaces. We discuss the use of the theory developed in recent years for describing the results of concrete experiments in modulation spectroscopy of surfaces. We especially examine the dependence of the surface-plasmon spectrum on the microscopic structure of the surface. We describe how one can obtain information from the data of optical measurements on the electronic structure of the surface, and in particular, we discuss the effect of adsorption on this structure. The review of the literature covers the period to the end of 1980.