The book is addressed to the advanced undergraduate and postgraduate students interested in lasers. It encompasses a vast area, joining laser physics and nonlinear optics. Earlier chapters give a concise account of introductory physics on which the operation of lasers depends. Quantum mechanical treatment of the interaction of radiation with an atom and some critical optical processes is presented. This prepares the way, by easy stages, for the formal development of lasers. The advances in laser physics over the last twenty-five years are described. The material in nonlinear optics scattered in various journals is collected compactly and unified. Some research problems of current interest are discussed. The treatment is kept as simple as possible so that students may not have any difficulty with it. The book will help the readers acquire new concepts and advanced methods in nonlinear optics and give them an overall perspective necessary to continue further independent research. The book will be beneficial to a variety of readers.