Abstract Chromogenic materials can be used in ‘smart’ windows to modify the incoming visible light and solar energy in buildings as well as for other see-through applications. Electrochromic (EC) windows seem to be ready for large-scale applications to provide energy efficiency and indoor comfort. This chapter attempts to give an introduction to various chromogenic technologies and products. EC technology is mainly covered with a detailed description of materials, devices used and commercial products. Finally, the energy-saving potential and the environmental profile of such glazing are discussed.