There are several proteins in mammalian cells that can metabolize hydrogen peroxide and lipid hydroperoxides. These proteins include four selenium-containing glutathione peroxidases that are found in different cell fractions and tissues of the body. This review considers the structure and distribution of the selenoperoxidases and how this relates to their biological function. The functions of the selenoperoxidases were originally studied in systems where their activity was manipulated by changing dietary selenium levels. More recently, molecular techniques have allowed overexpression of selenoperoxidases in cell lines and animals. Additionally, cellular glutathione peroxidase knockout mice have been used to investigate the functions of this protein. From this work it is clear that the selenoperoxidases are involved in cell antioxidant systems. However, they also have more subtle functions in ensuring the regulation and formation of arachadonic acid metabolites that are derived from hydroperoxide intermediates. The range of biological processes, which are potentially dependent on optimal selenoperoxidase activity in mammals, emphasises the importance of achieving adequate selenium intake in the diet.