Deflections of slip vectors of interplate thrust earthquakes from expected directions are used to estimate arc-parallel strain rates within the overriding plates at the world's major convergent plate margins. Arc-parallel extension strain rates, between 10−8/yr and 10−7/yr and significant at 2 standard deviations, are observed in the fore arcs of the Aegean, Aleutian, Mariana, Sumatran, southern Kuriles, New Hebrides, Scotia, and southern Central American (lat 8° to 12°N) subduction zones and the Himalayas. Northern Chile (lat 17° to 31°S) and Central America (lat 11° to 18°N) show arc-parallel compression. Available geologic and geodetic estimates of fore-arc slip and strain rates agree within a factor of two with slip-vector estimates. Arc-parallel strain in fore arcs is rapid enough to produce geologically significant effects, such as unroofing of high-grade metamorphic rocks and disruption of transported fore-arc terranes. Fore arcs deform even where convergence is perpendicular to curved margins, demonstrating that head-on subduction can produce a three-dimensional strain field.