Primary tectonic stress in the lithosphere is predominantly caused by lateral density variations within the Earth and associated topographical loading. When such a stress system caused by major sublithospheric density anomalies is intersected by a weak zone which cuts across the lithosphere, plate boundary forces develop and modity the plate interior stresses. In this paper, finite element analysis is used to model the stresses and plate boundary forces associated with subduction plate boundaries, with dipping and vertical slabs extending to about 270 and 400 km depths having the subduction fault both locked and unlocked. In such regions, several types of horizontal deviatoric stress may occur, including (1) local compression in the trench--arc region caused by the dense sinking slab and the associated surface downflexure; (2) plate interior tension which occurs when this compression is intersected by a weak subduction fault; (3) local tension associated with thickening of the crust at the arc and elsewhere; (4) local tension in the back arc region produced by the underlying low density upper mantle; and (5) downbending stresses in the subducting slab, thermal stresses, and transmitted ridge push, which are not included in the modelling here. A gradient from compression in the forearc to tension in the back arc can be modelled in terms of these stresses when the fault is partially locked. It is, however, the intersection of the local compression (1) by an unlocked or partially locked subduction fault that modifies the plate interior stresses and gives rise to the slab pull and trench suction plate boundary forces. The state of stress in the interior of the overriding plate is also crucially influenced by back arc spreading where this occurs. Plate boundary forces have been evaluated for each of the models. It is shown that slab pull and trench suction may be of comparable magnitude. ¿ American Geophysical Union 1992