This chapter provides numerical data describing the magnitude of the energy required to sustain a high standard of living. It includes a review of technical and environmental factors for energy sources that are potential replacements for fossil fuels and examines their fitness to supply the energy for a high standard of living on a world wide basis. Sources that have any likelihood of being used in the next 30 years are examined One source is selected as the most suitable for future energy and the rationale for its selection is provided.