The emphasis of this chapter is on XPS, one of the most used electron spectroscopy techniques in the domain of polymeric materials. The success of XPS lies in its possibility to analyze all elements except hydrogen, and the availability of a large amount of data of core level electron binding energies for several elements in diverse chemical environments. Moreover, it is generally non-destructive, quantitative and permits to determine overlayer thicknesses in the nanometer scale. For these reasons, XPS is now routinely used in all aspects of surface science in general and in polymer science and engineering in particular. As this contribution is intended for the nonspecialist who wishes to get acquainted with the technique and its applications to polymers, we shall first briefly discuss its historical background, basic principles and instrumentation. Thereafter, we will review applications to a selected polymeric system (e.g. polymer grafts, polymer blends, interpenetrated polymer networks. . .) taken from the literature and from the authors' research materials.