This paper describes the types of instruments needed in searching for radioactive ores and the various corrections that must be applied to make the instrumental readings useful. The most important factor in interpreting the results of gamma ray surveys is the recognition of anomalies. An anomaly of this type may be defined as radiation unexpectedly large for the associated field conditions. A knowledge of the characteristic radiation intensities associated with different rock types is an aid in the recognition of anomalies. The characteristic radiation intensities of a number of rock types are given in the paper. It is important to consider the effect of rock type, field conditions, weathering and climate in interpreting anomalies. The highly radioactive igneous rocks of the Front Range of the Rocky Mountains in Colorado may be the source of the uranium in the ore deposits farther west.