This chapter discusses the effects of ultraviolet (UV) radiation in polymers. Absorption of electromagnetic radiation is a necessary prerequisite for photodegradation. Polymers such as polyolefins, which theoretically should be transparent to ultraviolet (UV) light, nevertheless do absorb UV radiation due to the presence of impurities from several sources. In most practical applications, the compounding ingredients in the formulation and the processing operation itself yield sufficient chromophores to allow these polymers to absorb UV radiation and therefore to undergo photodegradation. Polymers irradiated in air with solar UV radiation invariably breakdown due to a combination of photodegradative as well as photo-initiated thermo-oxidative mechanisms. With polymers such as the rigid poly(vinyl chloride) formulations used in building materials the visible damage is primarily a result of photodegradation. Photochemical changes in the polymer invariably affect the physical, chemical, and mechanical properties of the material. These changes are often used to quantify the photodegradation process. The consequences of photodegradation, which are of relevance to applications of polymers, include discoloration of the surface, surface damage such as cracking or chalking, loss of integrity as evidenced by reduced strength or extensibility, changes in impact strength of the polymer, and changes in physical properties such as solubility. A wide range of analytical approaches is available to study the effects of UV exposure on polymers.