The San Andreas fault can be divided into locked and free sections. On the locked sections accumulated slip is released in great earthquakes. On the free sections slip is occurring continuously either aseismically or during smaller earthquakes. Stress drops during earthquakes can be estimated from the ratio of short to long period amplitudes and from surface strain. Surface heat flow may provide an upper bound on the absolute stress. The failure or yield stress must reach a maximum at some depth on the fault. This maximum may occur in the near-surface brittle zone or deeper in the plastic zone of the fault. The historic distribution of seismic activity provides information on the stress level. The accumulation of strain and stress on the fault can be predicted using elastic theory. It is necessary, however, to include the viscous coupling of the lithosphere to the asthenosphere in order to fully model the problem.