Energy is the currency of life. Eat more than you use and you store fat; use more than you eat and you won't last long. But understanding bioenergetics is more than just a matter of our waistlines; it is fundamental to life and the interactions of all living things with the environment. Understanding how organisms use energy is also key to the survival of many species during this time of dramatic change. How will altering wind patterns impinge on migrating birds? What sets the upper limits on the amount of energy that animals can produce? How can we combat metabolic syndrome and the obesity epidemic? These are fundamental biological questions that go well beyond the protein structures and reaction mechanisms that underpin bioenergetics. Fascinated by biological energy use, Raul Suarez at the University of California, Santa Barbara, USA, has drawn together a collection of review articles addressing the issue of bioenergetics from a wide range of perspectives, from the ecological to biomedical. ‘I wanted to come up with an issue that demonstrates the range of research activity that goes on and shows that this is a vibrant and exciting area of science,’ says Suarez. Teaming up with JEB Editor-in-Chief Hans Hoppeler to invite contributors from various backgrounds with an interest in bioenergetics, Suarez has drawn together a unique collection of review articles addressing ‘The Biology of Energy Expenditure’.