Groundwater radon monitoring work carried out in southern California by the University of Southern California since 1974 is summarized here. This effort began with a sampling network over a locked segment of the San Andreas fault from Tejon to Cajon and was later expanded to cover part of the southern Transverse Mountain ranges. Groundwater samples were brought back weekly to the laboratory for high precision scintillation counting. Needs for more frequent sampling and less labor prompted the development of an economical and field worthy instrument known as the continuous radon monitor. About 10 have been installed in the network since early 1980. The groundwater radon content was found to show anomalous increases (mostly at a single station) before a number of moderate and nearby earthquakes. Our work is hampered by a lack of large earthquakes that may have a regional impact on radon anomalies and by the complexity of the underground hydrological regime. To circumvent this difficulty, we have chosen to monitor only deep artesian wells or hot spring wells.