Most reactive oxygen species (ROS) can oxidize methionine (Met) residues of proteins to methionine sulfoxide (MetO). However, unlike the ROS-dependent oxidation of other amino acid residues of proteins (except cysteine residues), the oxidation of Met residues is readily reversed by the action of methionine sulfoxide reductase (Msr) that catalyzes the thioredoxin-dependent reduction of MetO residues of proteins back to Met. We summarize here results of studies showing that the cyclic interconversion of Met and MetO residues of proteins is involved in several different biological processes: (a) It is the basis of an important antioxidant mechanism for the scavenging of ROS. (b) It is likely involved in the regulation of enzyme activities. (c) It is involved in cell signaling. (d) It can target proteins for proteolytic degradation. Furthermore, a loss in the ability to catalyze the reduction of protein MetO to Met residues leads to a decrease in the maximum life span, whereas overexpression of this activity leads to an increase in the life span of animals. In addition, a decrease in Msr activities in brain tissues is associated with the development of Alzheimer's disease.