The concept, design, fabrication, and calibration of a new type of layered heat flux gage is described. Using thin-film microfabrication techniques the gage design is able to combine many of the desirable characteristics of other previous gage designs to produce a gage that has a very small size, high frequency response, and the ability to measure very high heat flux rates. The heat flux microsensor incorporates a differential thermopile, a set of 100 differential thermocouple pairs connected in series across a thin thermal resistance layer. The gage is fabricated on a model surface without the need for additional adhesives. The design configuration allows the thermal insulating layer to remain very thin, while still retaining the high output typical of much thicker gages. The small overall thickness of the gage, less than 2 µm, gives it a fast time response with the capability of measuring heat transfer in transient flow conditions. The combination of small thickness and small surface dimensions, 1 mm by 1 cm, gives the microsensor negligible flow and thermal disruption. The performance of a prototype microsensor deposited on Corning 7059 glass was measured. Steady-state calibrations were done using a convection calibration apparatus. The measured sensitivity (before amplification) was 0.164 mV per watt/cm². The output was linear over the range tested from 800 to 9000 watts/m². The unsteady response was tested using a continuous laser beam directed through a variable speed chopper wheel onto the gage surface. Results showed a frequency response of at least 1 kHz. / Master of Science