Laser physics and nonlinear optics are fields which have been intimately con­ nected from their beginning. Nonlinear optical effects such as second-har­ monic generation fulfil vital functions in many laser systems. Conversely advances in laser development quickly lead to progress in nonlinear optics. Of particular importance has been the development of tunable visible and uv lasers. With the ability to tune the laser frequency into close resonance with atomic transition frequencies, one can produce a large resonance en­ hancement of the nonlinearity. This permits the observation of a great var­ iety of nonlinear optical processes in dilute media such as atomic vapours. In recent years much of the research effort in nonlinear optics has been directed towards the use of such media, and it is this area which forms the subject of the present book. We review a wide range of nonlinear optical processes in atomic vapours, molecular gases and cryogenic liquids. At the same time we have tried to treat the subject in sufficient depth to be useful to research workers in the field. To achieve this, a measure of selectivity has been introduced by emphasising those nonlinear processes which are seen to have applications as sources of tunable coherent radiation. Thus we have not discussed in any detail those nonlinear processes whose main applications are in spec­ troscopy, such as Doppler-free two-photon absorption.