There are three well-described rotary biological motors: 1. The mitochondrial ATP synthase, which is responsible for conversion of a proton gradient into the “biological fuel” ATP. 2. The bacterial flagella motor, which can be either a proton-driven system or a sodium-ion-driven system. 3. The rotary DNA-loading protein found at the head of bacteriophages, which is responsible for ensuring nucleic acid, is “loaded” into the viral head. 3.1  ATP SynthaseAs well as being known for its uniquely universal role at the primary source of the “energy-currency” of biology — adenosine triphosphate (ATP) — ATP synthase has become particularly well known for two reasons (for a recent review, see reference [70]). The first is the observation that the motor can rotate with an efficiency approaching 100%,25 and the second is the elegant experiments that led to the visualisation of the rotary motion (Fig. 3.1). The possibility that ATP synthase was a rotary motor was first proposed by Boyer,71 but it was a long time before this suggestion was accepted. This eventually occurred when the atomic structure of the F1 section of the enzyme was solved in 1994 by John Walker and colleagues.72