Tungsten oxide is today the most widely used electrochromic material, and the optical and transport properties have been extensively studied. The optical properties of polycrystalline tungsten oxide films are well understood in terms of polaron theory. However, there is still a lack of detailed physical understanding of the properties of amorphous tungsten oxide. In this paper we review the concept of defects in amorphous materials with a special emphasis on tungsten oxide coatings produced by sputter deposition. Different types of oxygen vacancies and interstitials are believed to be of importance depending on the deposition conditions of the films. Tungsten oxide exhibits a strong electron-phonon interaction, which makes the occurrence of doubly charged defects likely. Different defects give rise to transparent and coloured as-deposited amorphous tungsten oxide coatings. We also discuss the character of the localized electronic states giving rise to the electrochromic effect upon proton or lithium intercalation. (C) 2004 Elsevier B.V. All rights reserved.