Deformation-induced dislocation densities, subgrain sizes, and grain sizes in rocks from deeply eroded fault zones provide estimates of the differential stresses that existed when the faults were active. Laboratory experiments have demonstrated that the dislocation density,subgrain size, and grain size generated during steady state deformation depend primarily on the magnitude of the applied differential stress; temperature has only a minor influence on these three quantities through materials parameters such as the shear modulus. Theoretical arguments support these experimental results. Differential stresses in fault zones, determined from the empirical stress-micostructure relations, lie in the range 40--200 MPa. In many fault zones, such as the Moine thrust zone in Scotland, the progressive development of the deformation-induced microstructure can be observed, particularly in quartzites. Far from the fault surface, quartz grains are nearly equant but show undulatroy extinction. As the central portion of the fault is approached, extinction remains undulatory as the grains become increasingly elongated (aspect ratios of 100:1 are common) and recrystallized. Quartizites are often 100% recrystallized near the centers of fault zones. This progressive development of the microstructure indicates that recovery at low or zero stress has not erased information on the last tectonic event in the plastic deformation regime.