This chapter discusses the use of energy during prehistoric times. Our direct ancestors spent their lives as simple foragers, and it was only about 10,000 years ago that the first small populations of our species began a sedentary existence based on the domestication of plants and animals. This means that for millions of years, the foraging strategies of hominins resembled those of their primate ancestors, but we now have isotopic evidence from East Africa that by about 3.5 million years ago hominin diets began to diverge from those of extant apes. The chapter first considers how bipedalism started a cascade of enormous evolutionary adjustments such as adaptations underlying tool use and adaptation to high-quality, energy-dense foods (meat, nuts) before providing an overview of foraging societies and the origins of agriculture.