This paper presents an overview of the operating principles and characteristics of acoustooptic and electrooptic diffraction light modulators. Functionally, these may be categorized into temporal light modulators, spatial light modulators, and tuneable color filters. Physically, they may be categorized by driving mechanism and by optical geometry. Acoustooptic modulators may employ traveling wave or standing wave ultrasonic fields to induce photoelastic modulation of the optical indices of refraction. Electrooptic modulators may use electrostatic fields or traveling electromagnetic waves. The three principal classifications based on optical geometry are bulk-wave, total internal reflection, and guided optical wave devices. In bulk optical devices, the optical beam interaction occurs within the active medium, largely unaffected by material interfaces. In total internal reflection devices, the optical beam can interact with a perturbation confined near surface of the active material, but otherwise maintains bulk propagation characteristics. In guided wave optical devices, the light is confined to a dielectric waveguiding structure whose optical index characteristics are further perturbed by the active modulation. For most diffraction modulators, the controlling physical principle is the conservation of momentum, which may be elaborated as wavevector matching or phase matching conditions. Spatial light modulators involve control of the wavevector components transverse to the optical propagation direction. Color filters affect the wavevector component along the optical propagation direction. Temporal modulators affect the amplitude or frequency of a light source considered as a whole: they may utilize transverse or longitudinal modulation; frequency shifts are fixed by the conservation on energy.