Abstract: The invention is a heat flux gage applied to a surface, for the measurement of surface heat transfer. All elements of the gage are thin films, deposited on the surface by sputtering or an equivalent process. The gage comprises a plurality of thermocouple junctions connected as a differential thermopile, with hot and cold junctions on the two faces of a planar thermal resistance element. Electrical insulating and protective layers are deposited between the surface and the gage, and over the exterior of the gage. The gage output signal is a voltage indicative of heat flux.