When an electroactive species undergoes a change in colour upon electron transfer (oxidation/reduction) the process is known as electrochromism. This process normally involves the passage of an electric current and is reversible. In the late 1960s, it was suggested that, because of their reversible change in colour, electrochromic materials could be used to make coloured displays. In this chapter the mechanism of electrochromism in cells will be discussed only briefly, followed by a more detailed description of the various classes of electrochromic materials, and finally an account will be presented of the burgeoning applications for these systems, e.g. commercial use of electrochromism in dimmable windows in automobiles and aircraft.