This book deals with the physical processes that take place in lasers. The author considers three groups of problems: how to get inverted active media, the formation of a radiation field in a resonator, and the dynamics of the processes in lasers. This work analyzes different directions in the development of lasers: dye lasers, compressed-gas lasers, recombining plasma lasers, unstable resonators, synchronization of longitudinal and transverse modes, etc. Also, the author presents a systematic examination of the methods, different approaches and approximations used in laser theory. The book is intended for scientists and engineers working in the field of laser technology, as well as teachers and students of physical sciences.