电致变色是材料的光学属性（反射率、透过率、吸收率等）在外加电场的作用下发生稳定、可逆的颜色变化的现象，在外观上表现为颜色和透明度的可逆变化。具有电致变色性能的材料称为电致变色材料，用电致变色材料做成的器件称为电致变色器件。

电致变色材料一个很好的例子是聚苯胺，聚苯胺可以通过电化学过程或者苯胺的化学氧化过程来形成。 如果把电极浸入含有低浓度苯胺的盐酸溶液中， 在电极上就会产生聚苯胺薄膜。根据不同的氧化态，聚苯胺可以呈现为浅黄色或者深绿/黑色。其它找到技术应用的电致变色材料包括紫罗碱和en:polyoxotungstate。更多的电致变色材料包括氧化钨 (WO₃)，它的主要化学用途是制作电致变色窗或者智能窗。

Electrochromism is the phenomenon displayed by some materials of reversibly changing colour by using bursts of charge to cause electrochemical redox reactions in electrochromic materials. Various types of materials and structures can be used to construct electrochromic devices, depending on the specific applications. Transition metal oxides are a large family of materials possessing various interesting properties in the field of electrochromism. Among them, tungsten oxide (WO₃), has been the most extensively studied material, used in the production of electrochromic windows or smart glass and more recently electrochromic displays on paper substrate as anti-counterfeiting systems integrated on packaging. NiO materials have been widely studied as counter electrodes for complementary electrochromic devices, in particular, smart windows. The world leading institutions on NiO efforts include National Renewable Energy Laboratory and Uppsala University. Another example of an electrochromic material is polyaniline which can be formed either by the electrochemical or chemical oxidation of aniline. If an electrode is immersed in hydrochloric acid which contains a small concentration of aniline, then a film of polyaniline can be grown on the electrode. Depending on the oxidation state, polyaniline can either be pale yellow or dark green/black. Other electrochromic materials that have found technological application include the viologens and polyoxotungstates.