The monograph is devoted to phenomena of nonlinear optics appearing on a macro­ scopic level in the interaction of intense light with an isolated atom. It is a first attempt to summarize the elementary phenomena of nonlinear optics and present the various methods used in experiment and theory. In essence, this book can be considered an expanded version of the new aspect of quantum mechanics and atomic physics that in time will be incorporated into te- books on this subject. By the middle of this century the interaction of light with atoms had become one of the most investigated branches of physics. However, in the mid-sixties the development of high-power lasers changed this situation completely. It is a well-known fact that lasers are essentially new sources of light with high intensity, sharp directivity, and practically ideal monochromaticity. Entirely new phenomena came up in the studies of the interaction of light with atoms. In an intense light field, multiphoton transitions become important. The field disturbs the atomic levels, shifting, broadening, and mixing them. In an extremely strong field the atom ceases to be a bound system. These and similar phenomena on the atomic (microscopic) level determine the variations in the averaged, macroscopic properties of the medium, variations that cause nonlinear-optics phenomena, which radically change the fundamental classical laws of the interaction of light with matter.