Mitochondria play a pivotal role in cellular metabolism and in energy production in particular. Predictably, defects of mitochondrial metabolism have a deleterious effect on cell function and survival, especially in highly energy-dependent tissues such as brain and skeletal muscle. Although a multitude of biochemical reactions occur within mitochondria, the oxidative phosphorylation (OXPHOS) system is the most important in terms of adenosine triphosphate generation and in its association with human disease.