We present the theory of Brillouin scattering for an interface composed by a semi infinite medium and a finite slab. According to the thickness of the slab and the nature of the two systems various interesting features are present in the scattering cross section. In backward scattering experiments, to which we refer, are present structures corresponding to the Rayleigh, Sezawa and Lamb modes polarized in the sagittal plane. By considering transparent thick films of sufficiently high thickness it is possible to obtain information on the interface mode, the Stonely wave. This mode gives rise to a maximum in the Brillouin cross section. In the limit in which the thickness of the slab goes to infinity, the Lamb modes give rise to the density of states of the supported semi infinite medium.