Energy presents one of the world's most complex and persistent challenges. Energy access and use is a necessary and powerful driver of socioeconomic development; on the other hand, the burning of fossil fuels for energy is altering the Earth's climate. Transitioning to a global-scale, lower-carbon energy system is expensive and difficult, and affects nearly every aspect of human activity. Innovations in the production and use of energy may offer cheaper, cleaner energy—but will require the right mix of policy incentives, research programs, and deployment efforts. These articles tackle the energy challenge from a huge variety of angles, offering insightful proposals for a more sustainable global energy system.