According to the now widely-accepted endosymbiont hypothesis, mitochondria are the descendents of a bacterium that was “captured” early in evolution by a proto-eukaryote. Accordingly, this organelle is about the size of a bacterium (i.e., between 1–10 microns), and has many bacteria-like features. The modern organelle contains about 1000 proteins, almost all of which are are imported from the cytoplasm as translation products of nuclear-encoded genes. Some of these proteins are required for the biogenesis and maintenance of the organelle itself, and include factors required for mtDNA synthesis, mtDNA replication, translation of mtDNA-encoded mRNAs, maintenance and integrity of organellar membranes, transport of small molecules and ions, and detoxification of oxygen-related free radicals. Most of the other imported polypeptides are “housekeeping” proteins. In fact, many of the cell’s basic metabolic functions are housed within the mitochondrion, including enzymes for the oxidation of carbohydrates, for lipid transport and for β-oxidation, for the tricarboxylic acid (TCA) cycle, for amino acid metabolism, and for the transport and removal of nitrogen-containing compounds. Perhaps the most important housekeeping function of mitochondria is the production of ATP via the respiratory chain/oxidative phosphorylation system. The respiratory chain (Fig. 1) consists of five complexes located in the inner mitochondrial membrane: complex I (NADH dehydrogenaseubiquinone oxidoreductase), complex II (succinate dehydrogenase-ubiquinone oxidoreductase, which contains succinate dehydrogenase (SDH), a TCA cycle enzyme, as one of its 4 subunits), complex III (ubiquinone-cytochrome c oxidoreductase), complex IV (cytochrome c oxidase, or COX), and complex V (ATP synthetase). Complexes I, III–V contain polypeptides encoded by both nDNA and mtDNA; complex II contains only nDNA-encoded subunits.