Scenarios emphasizing improvements in end-use efficiency tend to meet sustainable development goals, such as ensuring (almost) universal access to electricity, maintaining air quality, and limiting global average temperature increases. However, if efficiency gains turn out to be small, the world will become highly dependent on rapidly improving innovation and increasing the supply of “clean” energy. Another insight is that sustainable development pathways have been devised that exclude nuclear power, and carbon capture and storage (and its bioengineering variants), technologies that face great sociopolitical and technical challenges.