This book is the first book devoted to nonlinear optics that treats the subject simply and consistently, with concise yet thorough descriptions of all interrelated phenomena, from the microscopic to the macroscopic. Using a simple model of a time-independent interaction of monochromatic light with an atomic gas, it describes the elementary nonlinear processes that emerge for an isolated atom, the optical characteristics of the medium averaged over a large number of atoms and depending on the intensity of light, and the basic nonlinear optics phenomena observed in the propagation of an intense light wave through the medium. The analytical-theoretical description of nonlinear optics phenomena serves as the focal point for the subjects covered, which the authors have singled out as the main phenomena that qualititively distinguish nonlinear optics from the common linear optics of weak light fluxes. Includes a list of notations and a list of references designed to make it easy for readers to pursue specific areas of research.