Photodegradation and light stabilization are very important aspects of polymer aging. Polymer degradation includes different types of processes: thermodegradation, oxidation, acting of ozone, photodegradation, radiation, hydrolysis, mechanical degradation, and biodegradation. It is very important to know the mechanism of polymer degradation in order to select stabilizers against the degradation. This volume presents the analysis of achievements in the field of photodegradation of polymers. It includes, first of all, data of Russian investigators who have decisively contributed in the development of this field of knowledge (the schools headed by academicians V.V. Korshak, N.M. Emanuel, N.S. Enikolopov, A.A. Berlin, and many others).