About half the heat from continents is derived from the radiogenic decay of trace quantities of the unstable isotopes of uranium, thorium, and potassium in the continental crust. This contribution to surface heat flow is highly variable, however, typically resulting in a large scatter of surface heat flow values even in stable regions. Approximately 20 years ago, careful work by Robert Roy, Ed Decker, and Dave Blackwell with Francis Birch (Earth and Planetary Science Letters, vol. 5, pp. 1–12, 1968) showed that for well determined heat flow values in plutonic rocks, surface heat flow is linearly related to surface heat production in a given province. This relationship allows heat flow to be separated into two components: heat generated in the upper crustal zone enriched in heat-producing isotopes and heat from below this zone (the “reduced” heat flow). Art Lachenbruch quickly showed that the only distribution of heat production with depth that would satisfy this linear relationship under conditions of differential erosion is an exponential decrease with depth (in the Journal of Geophysical Research, vol. 73, pp. 6977–6989, 1968). The relationship has now been extended to include heat flow-heat production data from high-grade metamorphic rocks, and heat flow provinces have been defined in every continent except Antarctica. Two major problems remain, however: