The purpose of this paper is to review observations pertaining to the state of stress in continental crust, and their implications for the shear and normal stresses acting on large, seismically active faults. We focus particularly on what informa-tion about stress magnitudes and orientations can be obtained from borehole and seismological measurements, respectively, and on the caveats associated with inter-preting that information. Borehole data strongly suggest that the state of stress in the brittle crust is maintained by Andersonian fractures whose frictional failure is governed by Byerlee friction coefficients of 0.6–1.0 and hydrostatic fluid pressures. However, the large-scale geometry of the San Andreas fault, and other similarly large plate-bounding faults, implies a different mechanical behavior. In particular, the near-field (<10 km) axis of maximum horizontal compression lies at an angle substantially in excess of the 23–30° expected for an Andersonian strike-slip fault. Recent borehole measurements reveal that this geometry persists for the San Andreas fault to within a few hundred meters of the fault core. These observations suggest that the simple, Andersonian characteristics of the small fractures that pervade the continental crust and apparently maintain ambient tectonic stresses break down at plate-boundary fault scales.