A number of highly doped oxides - in particular Sn02 and In203 - have been reported to show high infrared reflectivity while being transparent to visible radiation. Their optical behaviour is based on metal-like properties i.e. high concentration and mobility of the conduction electrons and a semiconductor bandgap of about 3 eV or higher. Thin films of these oxides can be adapted for many applications and a few examples will be described. The performance of these filters strongly depends on the doping and preparation conditions. Coatings prepared by spray pyrolysis show excellent electrical and optical properties, the upper limits of which are being discussed on the basis of defect mechanisms in solids.