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The interaction of sound waves at microwave frequencies and electromagnetic waves at optical frequencies is denoted by the term Brillouin scattering. In the past, this type of scattering has been used for measuring elastic constants of materials. Interest in this subject has been renewed with the availability of coherent light from a laser and there are now a variety of problems where this interaction can be utilized. It can be used to modify an optical beam by either deflecting it in space or translating it in frequency. It can be used to monitor the spatial distribution of acoustic energy. Brillouin scattering from thermal sound waves can be used to determine the velocity of sound and attenuation in media where it is difficult to use more conventional means. With stimulated Brillouin scattering an intense light beam can generate coherent sound of great intensity. In this article we discuss each of these cases and calculate the interaction in terms of classical parametric equations of traveling wave systems. The familiar forms of parametric amplifiers and parametric oscillators are exhibited. Recent experimental results are summarized and serve to confirm the simple picture wherein the two wave systems are coupled together by the photoelastic and electrostrictive constants of the medium.