Electro-optic devices can be designed to modulate various properties of a light wave, including phase, polarization, amplitude, frequency, and even its direction of propagation. Research over the years has focused on developing materials and complementary device geometries that together can yield desirable device performance for a large number of applications. This chapter provides an overview of the fundamental modes of operation for devices that alter the optical properties of materials in a controlled way for achieving light modulation. A mathematical treatment of light propagation in anisotropic and electro-optically active media is presented to enable the device designer to quantify the modulation of light as well as the effects of slight misalignments of the structure on polarization and energy flow. Various classes of materials investigated for electro-optic devices are presented; these include inorganic crystals, organic crystals, liquid crystals, semiconductors, and dye-doped polymers. Possible device geometries include bulk, integrated optical waveguides, and optical fibers. In addition, performance criteria are defined, and a wide range of applications for these devices are summarized.