Transparent, electrically conductive films (TCOs) have been prepared from a wide variety of materials. These include oxides of tin, indium, zinc and cadmium, nitrides of titanium and chromium, and metals such as silver and gold. The physical properties of these materials are reviewed and compared. A figure of merit for a transparent conductor may be defined as the ratio of the electrical conductivity to the optical absorption coefficient of the film. The materials having the highest figures of merit are fluorine-doped zinc oxide and cadmium stannate. Physical, chemical and thermal durability, etchability, conductivity, plasma wavelength, work function, thickness, deposition temperature, uniformity, toxicity and cost are other factors that may also influence the choice of material for any particular application. The TCO materials are ranked according to each of these factors. The main applications of TCOs will be examined to see how these factors entered into the selection of the materials actually used.