Photochemical reactions are the main class of chemical processes. These processes provide for life on Earth, since due to photochemical processes, plants absorb carbon oxide from the atmosphere changing them into the 'meal' for all creatures on Earth. Together with 'good' photochemical processes there are some 'bad' as well. This book is about photodestruction and light stabilisation and is devoted to these processes; for example, the heterochain polymers. They are the main class of large-tonnage polymers, which are used for manning a great variety of polymer and composite materials (including nanocomposites).