One major problem associated with the applications of polymers is their instability to weathering. The formation of oxidation products provoked by UV light appears as the major cause of this instability. Understanding the mechanisms by which these products are formed has been the subject of intensive research for the last thirty years. In addition to the academic interest for understanding the nature and the mechanistic behaviour of these species in polymers, understanding the degradation of the functional properties on rational basis is most valuable for practical applications of polymeric materials.