This comprehensive synthesis of current medical and evolutionary literature addresses key questions about the role body fat plays in human biology. It explores how body energy stores are regulated, how they develop over the life-course, what biological functions they serve, and how they may have evolved. There is now substantial evidence that human adiposity is not merely a buffer against the threat of starvation, but is also a resource for meeting the energy costs of growth, reproduction and immune function. As such it may be considered as important in our species evolution as other traits such as bipedalism, large brains, and long life spans and developmental periods. Indeed, adiposity is integrally linked with these other traits, and with our capacity to colonise and inhabit diverse ecosystems. It is because human metabolism is so sensitive to environmental cues that manipulative economic forces are now generating the current obesity epidemic.