Oxygen-producing photosynthetic cyanobacteria first appeared about 2.7 billion years ago. The subsequent buildup of oxygen in Earth’s atmosphere during the next approximately 1 billion years opened the way for organisms to evolve the very efficient aerobic oxidation pathway, which in turn permitted the evolution, especially during what is called the Cambrian explosion, of large and complex body forms and associated metabolic activities. In eukaryotic cells, aerobic oxidation is carried out by mitochondria (stages II–IV). In effect, mitochondria are ATP-generating factories, taking full advantage of this plentiful oxygen. We first describe their structure, dynamics, and diverse functions, and then the reactions they employ to degrade pyruvate and make ATP.