Spectroscopic ellipsometry is currently widely used to describe the polarimetric response of structured media. The conceptional shortcomings of the commonly used Fresnel approach to predict and simulate ellipsometric data of structured media is discussed using numerical solutions of Maxwell's equations. Fresnel's relations and Snell's law are, strictly speaking, not valid anymore. We explain via simple physical models the effects occurring at non-horizontal interfaces between different materials. At such interfaces, evanescent fields occur and modify the still well defined complex reflectivities for s- and p-polarization. When the field vector projected on the sample surface is neither parallel nor perpendicular to the interface, the fields for s- and p-polarization couple and cross-polarization arises from the different spatial behavior of the evanescent fields. (C) 2016 Elsevier B.V. All rights reserved.