Surfical manifestations of the long-term evolution of plates are examined and used to derive constraints on the pattern of flow in the mantle. Generation and consumption of plates are not balanced around single plates, so the sizes of plates and the relative positions of ridges and descending slabs change, a complex return flow thus being required. At present a net flow should occur from the sites of much subduction toward the southern hemisphere, where sea floor spreading predominates. To allow the relative motions between descending slabs, masses in the mantle should be displaced at a rate which is several times larger than the rate of generation of lithosphere. The patterns of plate motions and of the return flow change considerably over periods of 100 m.y. The formation of instabilities occurs much faster than the cycling of mantle material. Hence the flow in the mantle cannot consist of regular steady state cells in which material moves in simple circuits and which are linked to plate boundaries. The deep flow is not parallel to the relative motions of overlying plates, so it is not two dimensional. The magnitude and nonsteady state of the sublithospheric flow make it doubtful that instabilities originating within the lithosphere alone drive the flow. This fact, together with the abrupt changes of plate motions, including jumping of ridges and continential breakup, suggests that instabilities at depth, e.g., mantle diapirs, are most important. The plate tectonic regime helps generate such instabilities in the mantle by disturbing the temperature distribution there.