Out of the growing number of so far known mammalian selenoproteins four are peroxidases. Their common catalytic mechanism involves redox shuttling of a selenocysteine residue in the active site, where it forms a characteristic catalytic triad with hydrogen-bonded tryptophan and glutamine residues. These peroxidases differ in tissue distribution, substrate specificity, regulation, responsiveness to selenium restriction, and likely in their biological role. Cytosolic glutathione peroxidase, which predominates in balancing hydroperoxide toxicity, rapidly declines in selenium deficiency. Prophylactic selenium supplementation is considered reasonable also in asymptomatic moderate selenium deficiency whenever a clinical condition known to cause oxidative stress has to be anticipated.