Proteins are sensitive to reactive oxygen species (ROS) such as hydrogen peroxide, superoxide and hydroxyl radicals which may be produced by the reaction between oxygen and free electrons released from the mitochondrial respiratory chain. Protein oxidation is associated with a loss of function, and oxidized proteins accumulate in cells during aging. Within proteins, cysteine and methionine, the two sulfur-containing amino acid residues, are most sensitive to ROS, but they are the only ones which can be reversed. The methionine sulfoxide reductases (Msr) system is able to repair methionine sulfoxide and convert it into reduced methionine in proteins. Age-related accumulation of oxidized proteins has been reported to be due, at least in part, to a decrease in degradation of the modified proteins, but protein repair systems also appear to be involved in this process. Furthermore, the Msr system has been shown to be important in cellular protection against oxidative stress and dysregulation of redox homeostasis associated with aging.