Large-scale mantle convection, to first order, is a system driven by interior density anomalies, modulated by variable plate thickness and extreme rheology variations at the top of the mantle. The rheological difference between oceanic and continental regions significantly influences the surface velocity. We apply a three-dimensional Newtonian viscous flow model to explain the large-scale present-day plate motions. The density anomalies are derived from seismic tomography and a slab model. With a viscosity difference of a factor of 30--60 between continents and oceans in the upper 90 km of the Earth, we are able to explain both the observed large-scale poloidal and toroidal plate motions. The viscosity difference between continental and oceanic regions has major control on both the poloidal-toroidal kinematic energy partitioning and the pattern of toroidal motion. Nonlinear rheology can help establish toroidal motion. Plate motions can be explained by assuming either layered or whole mantle flow. In order to match the amplitude of observed plate motions, the value of the reference viscosity (corresponding to that of between 400 to 670 km depth) is 1.6¿1021 Pa s for layered mantle flow and 3.2¿1021 Pa s for whole mantle flow. However, the predicted net rotation of the lithosphere, from both layered and whole mantle flow models, is very small and cannot account for the amplitude of the net rotation obtained from the plate tectonic models assuming a fixed hotspot reference frame.¿ 1997 American Geophysical Union