ATP synthases are rotary enzymes found in bacteria, chloroplasts, and mitochondria. These complexes produce the majority of cellular ATP in aerobic cells using energy from the transmembrane proton motive force established by the electron transport chain. In mitochondria, dimeric ATP synthase is essential for formation of the inner membrane cristae. While rotary catalysis in the soluble F1 region has been studied extensively by X-ray crystallography, the structure of the membrane embedded FO region remained elusive until recently. In the past few years, electron cryomicroscopy structures of mitochondrial, chloroplast, and bacterial ATP synthases have revealed the architecture of the FO region, helping to explain the mechanisms of proton translocation, dimerization of the enzyme in mitochondria, and cristae formation. These structures also show that ATP synthases exist in different conformational states, illustrating the flexibility and dynamics of the complex.