A set of forces on the lithospheric plates, including a driving basal shear stress, is deduced and found to be plausibly consistent with the possibility that large-scale, whole-mantle convection cells contribute significantly to driving the plates. Frictional and driving plate boundary forces are estimated, and a basal shear stress (BS) is assumed to complete the force balance on each plate. Boundary frictional stresses are assumed to be large (of the order of kilobars), as suggested by Hanks [1977>. All BS stresses of significant magnitude are roughly aligned with plate motions. The BS is the only available driving force for the South American and Eurasian plates, and is probably significant in dirivng most other plates. More generally, the results demonstrate that force models are possible which differ substantially from previous models.