Thin films of tin-doped indium oxide are widely used for transparent conductors. One application of In2O3:Sn (ITO) is transparent contacts for electrochromic electrodes. Opticalsdesign of electrochromic devices requires knowledge of the optical constants for each layer from the near ultraviolet and visible to the Inid infrared. Determination of the optical constants of the electrochromic layer cannot be made in isolation; a complete device or at least a half-cell including a layer of ITO is required to change the optical state of the electrochromic material. Measurements on ITO were made using variable-angle spectral ellipsometry, and spectral transmittance and reflectance. A series of structural models were fit to this data. The problem is complicated because of inhomogeneity in the films, variability in the manufacturing process, and sensitivity to environmental conditions. The spectral dependency was modeled by a single Lorentz oscillator and a Drude free-electron component. This data was then used as the basis for a model to extract the optical constants for a tungsten oxide electrochromic film.