ATP synthase is the universal enzyme that synthesizes ATP, the universal fuel that powers most cellular processes. This protein is constructed from two rotary motors acting in opposition, and operating by two different physical mechanisms. The F1 motor generates a mechanical torque using the hydrolysis energy of ATP. The Fo motor generates a rotary torque in the opposite direction employing a transmembrane protonmotive force. The two motors are connected by a flexible coupling, and each motor can be reversed: The Fo motor can drive the F1 motor in reverse to synthesize ATP, and the F1 motor can drive the Fo motor in reverse to pump protons. Thus ATP synthase exhibits two of the major energy transduction pathways employed by the cell to convert chemical energy into mechanical force. A physical analysis of the F1 and Fo motors can provide a unified view of the mechanochemical principles underlying these energy transducers.