Adapted in 1999 from a presentation by Robert A. Hefner III at the 10th Repsol-Harvard Seminar on Energy Policy in Madrid, Spain, on June 3, 1999. this article examines the history of various primary energy fuels, reports the abundance of the natural gas resource base and predicts the energy mix well into the 21st century, forecasting a continuing transition towards decarbonization and towards more decentralized, more mobile, sophisticated and highly efficient energy technologies fueled by the more clean and chemically simpler fuels of natural gas and hydrogen.