This paper documents an energy ladder that nations ascend as their per capita incomes increase. On average, economic development results in an overall substitution from the use of biomass to fulfill energy needs to energy sourced from fossil fuels, and then toward nuclear power and certain low-carbon modern renewables such as wind power. The results imply an inverse-U shaped relationship between per capita income and the carbon intensity of energy, which is borne out in the data. Fossil fuel-poor countries are more likely to climb to the upper rungs of the national-level energy ladder and experience reductions in the carbon intensity of energy as they develop than fossil fuelrich countries. Leapfrogging to low-carbon energy sources on the upper rungs of the national-level energy ladder is one route via which developing countries can reduce the magnitudes of their expected upswings in carbon dioxide emissions.