Electrochromic devices (ECD) are systems of considerable commercial interest due to their controllable transmission, absorption and/or reflectance. For instance, these devices are mainly applied to glare attenuation in automobile rearview mirrors and also in some smart windows that can regulate the solar gains of buildings. Other possible applications of ECDs include solar cells, small- and large-area flat panel displays, frozen food monitoring and document authentication also are of great interest. Over the past 20 years almost 1000 patents and 1500 papers in journals and proceedings have been published with the keyword “electrochromic windows”. Most of these documents report on materials for electrochromic devices and only some of them about complete systems. This paper describes the first patents and some of the recent ones on ECDs, whose development is possible due to the advances in nanotechnology.