The techniques in use today for the calibration of nuclear probes and some of the factors that influence calibration are discussed. Scientists and engineers have now proved the validity of determining the moisture content and density of engineering materials by means of radioactive gages. Even though the ability of the nuclear-type gage to measure moisture and density is generally acknowledged, there are still a number of questions that need answering. One such question involves an appropriate method of calibration and recalibration of the gage. Another question is whether the experimentally determined calibration curves remain valid with varying soil constituents. Initial work involved in the calibration of moisture and density probes required masses of soil that represented an infinite volume. Satisfactory portable standards have now been made to calibrate neutron moisture probes.