The biological functions of selenium (Se) are mediated by the expression of selenoproteins such as glutathione peroxidase. Recent progress in biochemical characterization of selenoproteins has elucidated new functions of newly identified and classical selenoproteins. Most selenoproteins are involved in protection against oxidative injury and in redox regulation of cellular events. On the other hand, mechanisms of anticarcinogenic effects of Se compounds appear to include not only the expression of selenoproteins but also pharmacological actions of Se metabolites.