Data from horizontal control surveys are used to determine the regional deformation pattern across the southern San Andreas fault system and the history of movements since ~1930. The interseismic straining is spread over a wide zone, with no sharp concentration across major faults and with significant deformation extending out to distances on the order of 100 km from the San Andreas fault. The maximum right-lateral shear straining parallels the trends of the northwest striking faults of the region and tensor rates of from 0.1 to 0.2¿10-6/yr are observed. A slight decrease in strain rate away from the San Andreas is indicated by the data, but shallow seismic slip on this fault alone is incapable of releasing all accumulated strains. The substantial residual deformation must then be accommodated by other means, and the range of possibilities includes (1) permanent inelastic deformation over a broad region (2) massive pre- and/or post-seismic slippage on the San Andreas beneath the brittle seismic zone, and (3) earthquake faulting on several subparallel active strands of the San Andreas system. Although available data are insufficient to exclude any of these possibilities, the diffuse seismicity and numerous active faults of the region suggest the third mechanism is the predominant one. Aseismic fluctuations in the rate of deformation have been inferred during the 1959--1974 southern California uplift, and long-lived, wide-ranging postseismic effects have been observed following several major earthquakes and are particularly well-documented for the 1940 El Centro shock (M = 7.1). Such effects are large and unexpected, complicate determination of the pattern of secular deformation, and are difficult to separate from suspected earthquake precursors. Determination of the rate of relative plate motion across southern California is made difficult by both the great breadth of the zone of secular straining and the observed irregularities in deformation rate.