Fossil fuels are the source of two-thirds of the World's electricity - and a major source of carbon dioxide emissions. Mitigating human disruption of the global climate, at a tolerable cost, will not be possible without shifting a large portion of electricity generation to options that emit little or no carbon dioxide. John Holdren and Nathan S. Lewis discuss how this might be achieved: by generating electricity from renewable sources, nuclear fission, nuclear fusion, and from advanced fossil-fuel technologies that can capture carbon dioxide and sequester it away from the atmosphere.