Reversible transitions in molecular structure, symmetry and energy banding are very common in solids and liquids. Such transitions are known to occur both gradually and discontinuously, but they are of greatest interest when they occur relatively abruptly. The transitions are induced electrically, thermally, with pressure or by exposure to UV-visible radiation. Materials that respond to these excitations with spectral changes, notably coloration in the visible range, are electrochromic, thermochromic, piezochromic or photochromic respectively. Some of the materials in these classes have been prepared as thin films, but they are relatively few in number. Thin films that have been shown to switch dramatically in the visible range, as well as the near-IR, and their properties, are reviewed here. Spectral switching of these films has been of interest to us because of the potential to control solar loading dynamically through transparencies in buildings and vehicles.