Rectification, inversion, modulation, harmonic generation. and hetero-dyning are all effects well known in the world of electronics as the essential properties of practical devices. These, and many other, related. effects have applications in optics, made possible by the development of the laser, a source of intense monochromatic light. In this article, from the viewpoint of the materials scientist, the basic phenomena and their applications are presented before the various materials in use in nonlinear optics are discussed.