Several electrochemical systems exhibit an intense color change when an electrochemical reaction is induced by an applied electrical field or current. The electrochemical and electro-optical properties of several such systems have been investigated. These systems are termed electrochemichromic and they can be classified into the following categories: (i) simple redox reaction; (ii) redox reaction coupled with an independent chemical reaction, resulting in variable color persistence; (iii) redox reaction coupled with a chemical reaction producing an insoluble colored species; (iv) redox reaction wherein a solid film deposited on one electrode is reduced and exhibits an intense color change. Analytical models which can describe the electrochemichromic behavior are presented for the different categories. © 1975, The Electrochemical Society, Inc. All rights reserved.