Even with concerted efforts to exploit energy-efficiency opportunities and other demand-side solutions, the world’s energy needs are enormous and almost certain to continue growing as developing economies industrialize and as rising standards of living in many societies lead to increased demand for modern consumer goods, services, and amenities.

For most of human history, animals and biomass supplied the vast bulk of human energy needs. With the advent of the Industrial Revolution roughly two centuries ago, humans began to turn increasingly to hydrocarbons as their primary source of energy, marking a profound shift that brought with it an era of unprecedented technological, socio-economic, and cultural change. Today, as concerns about environmental sustainability and energy security mount, the necessity of a third transition—to a new generation of energy supply technologies and resources—seems increasingly inevitable, if still not quite imminent. Even as the world remains largely dependent on coal, oil, and natural gas, early elements of that transition are beginning to come into view.

This chapter reviews the supply-side energy technologies and resources that are likely to play a role in the transition to a sustainable energy future. Separate sections cover fossil fuels, nuclear power, non-biomass renewable resources, and biomass energy. In general, the focus is on supply-side solutions that could make an appreciable contribution to meeting world energy needs in the next 20 to 40 years. Longer-term options, such as nuclear fusion, methane hydrates, and hydrogen (as an energy carrier) are discussed briefly but do not receive extensive treatment here.