The elect rochromic effect based on the reversible electro-deposition of a viologen radical cation has been applied to information display systems. Problems of multiplexing can be overcome by the use of an auxiliary switch, such as a thin-film transistor. The question of speed is the subject of this paper, which discusses the factors influencing the contrast ratio of an electrochromic display. The contrast ratio is determined by electrochemical and optical considerations, the former governing the amount of material deposited and the latter the visual effect of the deposit. Electrochemically, the deposition may be influenced by a variety of mechanisms, including diffusion, migration, electrode kinetics and deposit resistance. Multiple pulse driving gives increased speed in diffusion-limited cases. Optically, the perceived contrast is maximised by increasing the absorbance of the deposited material and by optimising the diffuse reflectance of the display electrode. © 1979 AIME.