Backthrusting of the rear of the accretionary wedge, arcward over the rocks of the forearc basin, is a common aspect of accretionary tectonics. An older assumption that backstops of accretionary wedges dip back toward the arc, and that accreted material is emplaced below such backstops, is contradicted by observations from many active forearcs that indicate (1) backthrusting is common, (2) forearc basins are nearly ubiquitous associates of accretionary wedges, and (3) forearc basement, where imaged, appears to diverge from the sedimentary package, dipping under the wedge while the overlying sediments are often lifted up against it. Backthrusting may be favored where relief is high between the crest of the wedge and the surface of the forearc basin because the relief must be supported by shear stress along the backthrust. We find ratios of Lt/Lb averaging 2.64¿0.8, where Lt is the horizontal distance from the crest of the wedge to the front of the toe, and Lb is the distance from the crest of the wedge to the surface projection of the backthrust. This ratio is predicted to increase with total convergence and with sedimentation, because the toe undergoes higher deformation rates than the backthrust, and sedimentation in the trench favors outbuilding of the toe, whereas sedimentation in the forearc basin tends to cancel the gravitational component of the driving stress of the backthrust. ¿ American Geophysical Union 1988