Although it is estimated that half of all polymers produced are required for exterior use, the photochemical instability of most polymers (many degrade over a period of months) severely limits their outdoor application. In order to improve polymer photostability the mechanisms of photodegradation and environmental ageing must first be understood. All the available information on polymer photodegradation and photo-oxidative degradation is reviewed in this book. Practical information on how to perform experiments to study photodegradation in both laboratories and practical exterior conditions are given based on the author's 25 years of experience in the field. The photodegradation of most polymers is described in details as is the degradation caused by metal salts and metal compounds. Special attention is given to photodegradation in extreme conditions, such as ablative photodecomposition, photodegradation during a thermonuclear blast and photodegradation in the space environment. The approach taken is both theoretical and practical and the book should be of interest to engineers, technicians and researchers in the polymer and plastics industries, in particular those concerned with the photoageing or photostabilization. This book should be of interest to all those in polymers and plastics industries; scientists involved in photodegradation in industry and academia; senior undergraduate and graduates in photodegradation; and those dealing with photostabilization and weathering.