Measurements of stress changes using IRAD vibrating-wire Stressmeters suggest that significant stress changes have occurred in a large area of Southern California near active faults in the past several years. Long-term changes in stress follow a pattern of horizontal compression in approximately a NS direction relative to stress levels in the EW direction, in good agreement with the tectonic pattern established by youthful geologic structures, independent stress and strain measurements, and the recent seismic history of the area. The sensors are installed at a depth of approximately 20 m by wedging them against the walls of small-diameter boreholes, producing an initial 'prestress' of 0.5--2 MPa-20 bar). Consequently, they only measure changes from an initial prestress level and are not capable of determining absolute magnitudes of the stress field. By revising the standard procedures for installing and reading the sensors, we have improved the resolution level of the sensors when installed from approximately 15 kPa (150 mbar) to better than 3 kPa (30 mbar). Three sites are now located along the San Andreas fault between Elizabeth Lake and Wrightwood, and four sites are being moniotred along the Sierra Madre fault system at the southern margin of the San Gabriel Range. A new net along the San Jacinto fault between Hemet and Anza is now established but will require a year or more of monitoring before providing a useful long-term data sequence. Short-term anomalies are superposed on the long-term trends and may be useful for site-specific earthquake prediction purposes. In one case, a short-term anomaly was associated with a moderate earthquake 15 km from the measurement array. The stress-change measurements may also provide useful data on near-surface thermoelastic effects and on the symmetry of the earthquake source mechanism.