This book describes the physical properties of silicon of interest from the point of view of nonlinear optics. It begins with coverage of the foundations of the phenomenological and microscopic approaches used in the theory of optical harmonic generation in silicon. Then, the authors provide a complex model of second harmonic generation at silicon phase boundaries, taking into account the interference of various contributions to the nonlinear optical response, the anisotropy of the medium, the photoproduction of nonequilibrium carriers and much more. They supply a detailed analytical review of experimental studies showing both the history of nonlinear optics of silicon and silicon nanostructures and its current state.