This paper reviews recent work on electrochromic W-oxide-based films and devices Computed data an presented for electronic structure and optical properties of crystalline WO3. Techniques for making heavily disordered W oxide films are surveyed with emphasis on high-rate sputtering of porous layers. Ion intercalation is discussed with particular attention to modelling of Coulometric titration curves. Optical properties of disordered films are covered. especially as regards the saturation of the coloration efficiency at high intercalation levels and the enhancement of the coloration efficiency in substoichiometric films. The paper covers data on devices of all-solid-state type as well as being laminated polymer-based constructions. It is pointed out that a bleached-state transmittance up to similar to 80% has been reported several times and that flexible devices have been made on polymer foils.