The technology of optical switching devices for dynamic glazings and other applications is discussed. The status of the state of the industry and its developments is detailed. The technical emphasis will be on the properties of chromogenic materials covering electrochromic, phase-dispersed liquid crystals and dispersed-particle systems. Such technologies can be used for dynamic control of solar energy through building and vehicle glazings. Both visible and solar control can be obtained from these devices. Switching ranges can be from as high as 80% to 5-10% transmittance in the visible region. The energy and daylighting benefits of such glazings are discussed. Also, chromogenic glazings can be used for other product applications such as the modulation of reflector surfaces and large electronic information display systems. The science of selected electrochromic devices is covered. Technical issues concerning large-scale chromogenic devices are discussed.