A fundamental problem in geoscience research concerns the thermal regime of the earth, including temperature distributions with depth, sources and sinks of heat, and processes of heat transport. Measurements of heat flow at the surface of the earth, as well as those of heat production in surface and near-surface rocks, provide some useful insights to the thermal structure of the interior. Perhaps the most significant of these is that radioactive heat-producing elements (HPE) are not distributed homogeneously in the earth but, rather, are concentrated in continental crust, apparently by the same geochemical processes that govern crustal differentiation.