In recent years a number of non-linear Raman spectroscopic techniques have been substantially developed and are now proving to be powerful methods for the solution of many problems not only in spectroscopy but also in chemistry, physics and biology. These techniques include hyper Rayleigh and hyper Raman spectroscopy, coherent anti-Stokes Raman Spectroscopy (CARS), Raman Gain and In­ verse Raman Spectroscopy, Photoacoustic Raman Spectroscopy (PARS) and the Raman Induced Kerr Effect (RIKE). Hyper Raman spectro­ scopy although experimentally difficult is valuable for investi­ gating transitions which are not active in the infrared or in the linear Raman effect; and the other non- linear Raman effects can provide