This report outlines the changes in energy supply that will be required over the next fifty years. I describe the ultimate objective of controls on greenhouse-gas emissions and set a stabilization target for greenhouse-gas concentrations that is designed to achieve this objective. I translate this target into limits on the emission of carbon dioxide and the burning of fossil fuels over the next century, and estimate requirements for carbon-free energy supply over this period. Finally, I describe options for achieving this transformation in world energy supply and outline near-term research and development priorities.