The electrochromism of thin films of tungsten oxide has been studied extensively and the change in the optical properties upon electrochemical reduction has been attributed to the formation of polaronic states or an increased concentration of conduction electrons. Polarons are electronic states in which charge carrier localization is associated with a local lattice distortion. These localized states can hop between neighboring sites of differing valence through the absorption of a photon. The energy required for the charge-transfer between neighboring tungsten octahedra leads to the visible absorption of reduced tungsten oxides. Localized states can form near defect sites such as oxygen vacancies or interstitial cations formed during intercalation. For materials such as tungsten oxide with multiple different interstitial sites that cations can occupy, it is not well understood if specific sites favor charge localization or if their occupation introduces shallow donors that contribute free charge carriers to the conduction band.