Synthesis of adenosine triphosphate (ATP) in mitochondria is accomplished by a large molecular machine, the F1FO ATP synthase. Proton translocation across the FO region that spans the mitochondrial inner membrane drives ATP synthesis in the F1 region through a rotational mechanism. Guo et al. present a high-resolution structure of the dimeric FO complex from Saccharomyces cerevisiae, determined by electron microscopy. The structure gives insights into how proton translocation powers rotation and suggests how FO dimers bend the membrane to give mitochondria their characteristic cristae.