This chapter presents a preliminary qualitative and clear description of the basic properties of parametric and polariton light scattering. When a beam of light passes through transparent homogenous material—whether gas, pure liquid, or a perfect crystal—a small part of the light's energy is scattered in all directions by the atomic structure of the material. Parametric scattering is attributable to the decay of one or two incident photons into a pair of photons with altered frequencies and directions. A characteristic feature of parametric scattering is the common nature of both waves—the scattering and observed waves— are equivalent. Coherence also characterizes other types of scattering, if they occur through collective excitations of matter with a nonzero propagation velocity. It should be emphasized that coherent scattering involves the addition of the amplitudes of the partial waves radiated by individual molecules of the sample.