Book excerpt: Cooperative Effects in Optics: Superradiance and Phase Transitions presents a systematic treatment of the modern theory of cooperative optical phenomena-processes in which the behavior of many-body systems of radiators or absorbers is essentially determined by their collective interactions with each other. The book focuses on the theory of collective spontaneous radiation (superradiance) and provides a detailed physical explanation of the mechanism of collective spontaneous emission. It considers numerous models of novel nonequilibrium light-induced phase transitions in a typical quantum electronics system, including two-level atoms interacting with the radiation field and more complex systems of three-level atoms, two-bank semiconductors, and other interatomic interactions with the electrostatic and lattice displacement fields. The book uses some of these models for the interpretation of experimentally observed light-induced critical phenomena. Cooperative Effects in Optics is of great value to research workers in the field of cooperative optical phenomena, especially in the determination of the physical essence of theoretical models developed to describe cooperative effects in multi-atomic systems.