The lunar heat flow experiment measures heat flow, temperature, temperature differences, and thermal conductivity in the shallow subsurface layer of the moon with heat flow probes. Through thermal analysis, precise calibration, and extensive tests, probes were developed. to measure the ambient temperature in the lunar surface layer to ± 0.1 °K over the range of 200 to 250 °K, temperature differences of 0 to 2°K-across 0.5 meters with an accuracy of ± 0.002°K and a stability of ± 0.002°K per year, and thermal conductivities over the expected lunar range-of 2 × 10^-3 to 4 × 10^-1 W/m°K to within 20%. This paper describes the principles underlying the experiment, the heat flow probe design, and measurement and test techniques.