Reviews the nonlinear optical properties of condensed matter. The non-linear optical susceptibilities are first introduced in a phenomenological manner and the effects they describe are presented on general grounds. Then the symmetry aspects of non-linear optics are discussed. The propagation of electromagnetic fields in non-linear media are considered and the device applications they result in are described. Finally, the origins of the non-linear optical behaviour of matter, either in the transparency domain or near energy level resonances are examined.
