The observed decrease of continental heat flow with tectonic age is interpreted in a three-component model. The first component is radiogenic heat from the zone of crustal isotopic enrichment, which yields about 40% of the observed heat flow in terrains of all tectonic ages. Because the surface heat flow varies with age between about 90 and 45 mW m-2, this radiogenic component also varies with age between about 36 and 18 mW m-2, a net decrease of some 18 mW m-2. The absolute decrease with time of this component is achieved by erosional removal of radioisotopes from the surface; the erosion, exponentially decreasing with age, has a characteristic time of some 300--400 Ma (m.y. B.P.). The second component of surface heat flow is residual heat from a transient thermal perturbation associated with tectogenesis. This transient yields approximately 30% of the heat flow (27 mW m-2) in Cenozoic tectonic zones, diminishing effectively to zero in late Precambrian to mid-Precambrian terrains. The decay of this component has been fitted with temperature perturbation models that show a maximum perturbation to a cool shield geotherm of 750¿--800¿C at a depth of 80--100 km. From the analysis of this transient component we conclude that for residual heat to be observable up to and beyond 500 Ma, conductive cooling likely extends at least to depths as great as 300--350 km, implying that the continental crust and upper mantle beneath 500-Ma and older terrains has remained as a single conductive unit to such depths for at least the age of the terrain. For younger terrains the residual heat is yet sufficient to weaken them and make them vulnerable to recurrent tectonism. The third component of surface heat flow is a background flux of 27 mW m-2. Some 15 mW m-2 arises within the thick continental conductive root and is likely to be radiogenic, although the isotopic concentrations required imply some modest posttectonic metasomatic enrichment. The remaining 12 mW m-2 of the background comes from deeper within the earth, perhaps from the core.