REDUCED continental heat flow, defined as the measured heat flow minus that due to radiogenic sources in the upper crust, initially decreases with time after the last thermotectonic event in a region(1), but then tends to stabilize at a roughly constant value, which is higher than would be predicted by a conductive cooling model (Fig, 1). Here we argue that this behaviour results from the influence that continents have on mantle heat loss, The finite thermal conductivity of continental crust and the fact that it is not recycled into the mantle on a large scale, as is oceanic crust, allows stable continental blocks to limit the local heat flux out of the mantle, Numerical models that allow blocks of crust to form over a mantle layer demonstrate how thermal perturbations associated with mantle convection penetrate into continents, causing lateral temperature variations at their base that parallel those in the mantle just below, This tends to establish a constant flux of heat from the mantle into the continents, which accounts for the trend in reduced heat flow.