OVER THE NEXT SEVERAL YEARS, electrochromic mirrors will be introduced for use on automobiles, initially as interior mirrors and later for the exterior. Electrochromic mirrors have variable reflectivity and this offers an opportunity to select a reflectance level that avoids glare, but that maintains rear vision. Human factors studies can relate driver discomfort to incident glare and the results can be incorporated into a fully automatic mirror whose reflectivity varies dynamically to suit changing driving conditions. In automatic mode, the electrochromic mirror responds appropriately to all driving situations, and mirror operation is transparent to the driver with smooth response and no unexpected change. This paper explains the scientific basis of electrochromism and traces development of the technology to its present applications. Electrochromic mirrors presently available worldwide are reviewed. The performance desired of an electrochromic mirror is defined and the ability of current electrochromic mirrors to achieve this performance is discussed.