Summary Mitochondria are the tiny descendants of bacteria that inhabit each of your cells. We need mitochondria (they provide us with energy) and mitochondria need us (we provide them with oxygen and nutrients). Mitochondria generate your cells’ energy. In an orchestrated series of chain reactions, mitochondria generate ATP, the chemical driving force behind cell function, muscular power, and cardiac output. The vast majority of human energy comes from ATP generated within mitochondria, which are therefore the key in understanding athletic performance. This chapter explores mitochondria from a number of perspectives: structure, origins, biochemistry, and genetics. The final step in the production of ATP involves the most intricate enzyme known in biology. Driven by the proton battery inside each mitochondrion, ATP synthase churns out ATP using a high-revving nanomotor mechanism. Whereas most other enzymes are made of a single protein, ATP synthase is a complex of 31 proteins, with several parts analogous to the machinery in a high-performance automotive engine. Variations of this enzyme give vitality to virtually every form of life on Earth. It is also the athlete’s engine.