The hypocentral regions of 29 (M ≥ 3.5) earthquakes along the San Andreas fault system in central California have been systematically examined for increased microearthquake activity preceding the main shock. Of 20 shallow (h > 9 km) main events, 10 were preceded by an apparent buildup of seismic activity. None of 9 deeper (h < 9 km) main shocks was preceded by increased activity. The early buildup of seismicity typically began days to weeks before the main shock, was followed first by a period of quiet, then by immediate foreshocks within 24 hours of the main event. Three main shocks with no significant early buildup in the level of seismicity were preceded within 24 hours by single events that, in retrospect, might also be considered foreshocks. Few of the events on the San Andreas fault system examined here were preceded by well-defined foreshock sequences like that associated with the 1975 Oroville earthquake.