Energy has long been a basic requirement of human societies. Archaeological evidence from caves in Africa suggests that fire was used by early humans, the hominids, as long ago as 1.5 million years BP, and fire has since fuelled the technologies on which civilized societies have been built. Today, regular supplies of energy drive the motors, appliances, cities, industries and transport on which the lifestyle of most of the Earth’s human population depends. Through time, the ability of human cultures to access energy, and the amounts of energy used, are often seen as indicators of society’s level of development and resource use. Energy use’s attendant environmental problems have long been with us, however. One of the first examples of environmental legislation in England occurred during the reign of Edward I when coal burnt for industrial and domestic purposes caused so great a smoke nuisance that the nobility, strongly backed by London residents, were able to obtain a royal proclamation forbidding coal burning in 1306 (although it proved impossible to enforce). Since then, the generation of energy and the use of fuels in multifarious ways to drive the industries and machinery of modern societies have facilitated an unprecedented level of both deliberate and inadvertent impacts on the global environment. The production, transportation, conversion and use of energy, particularly that derived from fossil fuels, are responsible for some of the world’s most serious environmental problems. These include global climatic change (see Chapter 11), acid rain (see Chapter 12) and pollution from motor vehicles (see Chapters 10 and 16), but human society’s harnessing and use of energy lies behind virtually every one of the environmental issues found in this book. This chapter will focus on energy sources, specifically on the possible future of those not based on fossil fuels.