The basic aspects of electrochromism induced by a p-doping/undoping process of conjugated polymers are briefly described. The most important requisites for an electrochromic device are discussed, and polymer electrochromic performance data as well as the test results of a polymer-based variable light transmission electrochromic device are reported. The colour contrast control of conjugated polymers by “tailoring” their conjugation length is discussed and spectroelectrochemical data of poly(3-methylthiophenes) electrosynthesized from 3-methylthiophene isomeric dimers and isomeric tetramers are reported.