ACCORDING to the theory of plate tectonics, relatively rigid plates are bounded by large faults; plate motion has negligible internal strain(1,2), with significant toroidal component to the velocity(3). By contrast, models of mantle flow with viscous rheology in an intact medium predict little toroidal component and substantial internal strain in surface motion(4). It has been suggested(5) that the observed characteristics of plate motion are related to faulted plate margins, which are observed to be weak(6). Here we confirm this suggestion, using three-dimensional models of mantle flow that incorporate faults and the forces exerted on plates by subducting slabs (’slab pull’) and mid-ocean ridges (’ridge push’). Our models show that plate-like motion results from the interaction between weak faults and a strain-weakening power-law theology. Weak transform faults tend to guide plate motion. This guiding effect and the decoupling that occurs at thrust faults may result in oblique subduction. Convergent margins are associated with realistic trench and fore-bulge topography. By simultaneously predicting surface kinematics, topography and gravity, the models achieve a useful degree of tectonic realism.