Mitochondria are essential for cellular function, providing ATP for energy and regulating a number of key metabolic processes including apoptosis and calcium homeostasis. The consumption of oxygen by the mitochondrial electron transport chain leads to production of ATP but also to release of electrons that can lead to the generation of free radical species and oxidative damage. Thus, proper function and control of mitochondrial is critical. Mitochondrial function has long been thought to be compromised during aging leading to alter physiologic function and age related pathologies. Our work on sarcopenia described above suggests that altered mitochondrial function and increased generation of reactive oxygen species can contribute to the physiologic decline seen in sarcopenia and ALS. However, in other studies we have shown that altered mitochondrial function can have beneficial effects on metabolism. For example, mice lacking Surf1, a mitochondrial electron transport chain protein assembly factor, show compromised assembly of complex IV and induction of a mitochondrial homeostasis pathway, the mitochondrial unfolded protein response (mtUPR) that includes induction of a number of mitochondrial chaperones and proteases such as hsp60, Clp protease and potentially Lon protease. Despite a more than a 50% reduction in Complex IV activity these mice are active and viable and show increased insulin sensitivity. Our current studies are investigating the role of these mitochondrial proteases during aging and metabolism as well as the potential link between loss of Surf1 and enhanced metabolism using studies in cultured cells and whole animals.