The paper deals with the electromangetic theory of the optical interactions in planar multilayer (ML) structures displaying magnetic order. The main attention is focussed on the practically important configuration of normal incidence and polar magnetization in an originally isotropic medium. The procedure providing the information on the ML optical response is treated in detail. It assumes that the thickness and optical parameters in each layer entering the ML structure allong with the azimuth and ellipticity of the incident wave are given as the input data. The optical response is given in terms of the complex amplitudes of the reflected and transmitted waves from which the ML optical parameters, i.e. “Kerr” and “Faraday” effects, transmission, reflectivity, quality factor, etc., can be derived. The experience has shown that the procedure gives plausible results even for ML structures consisting of the layers the thicknesses of which are in the nanometer range, i. e. in the range where the interface effects play an important role.