Abstract The results of theoretical and experimental investigations of the quantum size effect in thin films are presented. The locations of the extrema of the size sub-bands and of the Tamm surface states are calculated as functions of the film thickness. A new length R is introduced which characterizes the crystal surface properties and allows the behaviour of the Tamm levels and of levels connected with the size quantization to be determined. Experimental results for the tunnel effect in the size-quantized film-dielectric-metal system, for conductivity oscillations in quantizing magnetic fields normal to the film and for magneto-optical absorption are discussed. It is shown from these experiments that new information about the electron spectrum in solids can be obtained using the quantum size effect, i.e. without using a priori theoretical models it is possible to determine the electron dispersion law in a direction normal to the film and to determine the topology of the isoenergetic surfaces located both above and below the Fermi level.