A phenomenon observed in the scattering of light as it interacts with a material medium in which the incident light suffers a change in frequency due to internal energy change of the molecular scatterers. Raman scattering differs in this respect from Rayleigh scattering in which the incident and scattered light have the same frequency. Shifts in frequency are determined by the type of molecules in the scattering medium, and spectral analysis of the scattered light can provide a “fingerprint” of the chemical structure of the scatterers. Both incoherent and coherent forms of the Raman effect exist. Spontaneous Raman scattering, the usual (incoherent) form, is very weak, with Raman signals 4–5 orders of magnitude smaller than Rayleigh scattering and about 14 orders of magnitude smaller than fluorescence. In stimulated Raman scattering, a coherent form, the signals may be quite large. In addition, there are nonlinear forms of Raman scattering, including hyper-Raman scattering and coherent anti-Stokes Raman scattering (CARS).