Earthquake focal depth distributions and effective elastic thickness variations on the continents suggest that in old cold regions such as shields, the lower continental crust can be stronger than the upper mantle, and imply that the upper mantle can flow even when the lower crust does not. However, in other places substantial flow in the lower crust does occur, particularly near extensional metamorphic core complexes. We suggest that the lower crust only flows when its viscosity is significantly reduced, either by heating from igneous intrusions or by the addition of water. In both cases, the important characteristics of the flow are a consequence of the limited time over which the viscosity reduction occurs. The timescale is controlled either by conductive cooling of intrusions, or by the separation of melt from its matrix, which removes the water.