The Earth can be viewed as a massive heat engine, with various energy sources and sinks. Insights into its evolution can be obtained by quantifying the various energy contributions in the context of the overall energy budget. Over the past decade, estimates of the heat flow across the core-mantle boundary, or across a chemical boundary layer above it, have generally increased by a factor of 2 to 3. The current total heat flow at the Earth's surface - 46 +/- 3 terawatts (1012 J s-1) - involves contributions from heat entering the mantle from the core, as well as mantle cooling, radiogenic heating of the mantle from the decay of radioactive elements, and various minor processes such as tidal deformation, chemical segregation and thermal contraction gravitational heating. The increased estimates of deep-mantle heat flow indicate a more prominent role for thermal plumes in mantle dynamics, more extensive partial melting of the lowermost mantle in the past, and a more rapidly growing and younger inner core and/or presence of significant radiogenic material in the outer core or lowermost mantle as compared with previous estimates.