In eubacteria, chloroplasts and mitochondria, the synthesis of ATP is carried out by a highly complex molecular machine known as ATP synthase. Our aim is to understand how this machine works. We are concentrating mainly on the enzyme from mitochondria which has many features in common with the bacterial and chloroplast enzymes. It sits in the inner membranes of the organelle, where it uses the transmembrane proton motive force (pmf) generated by the oxidation of nutrients as a source of energy for making ATP. The pmf across the inner membrane of the organelle is coupled to the chemical synthesis of ATP from ADP and phosphate by a rotary mechanism illustrated in the Figure. During ATP synthesis, the central rotor turns in the direction shown about 150 times every second. In order to provide energy to sustain our lives, every day, each one of us produces a quantity of ATP by this mechanism that is approximately equal to our body weight.