Selenium-dependent glutathione peroxidase (SeGPX, EC 1.11.1.9) acts as part of the antioxidant defence of the body. It functions as a highly efficient catalyst to reduce a wide variety of intracellular peroxides, including hydrogen and lipid peroxides, thereby detoxifying these potentially damaging molecules. Thus selenium has a critical role in normal cellular biochemistry and the proposal has been made that deficiency of selenium in the diet can lead to a number of clinical disorders. World-wide interest in selenoproteins is now evident. In general, the active site of SeGPX and of other selenoproteins is based upon a selenium moiety (generally a single covalently bound atom of selenium) present as the amino acid selenocysteine. There has been an extension of the genetic code to include the codon TGA (in the correct DNA context) as the 21st codon specifying the presence of selenocysteine in the polypeptide structure of selenoproteins. Recent discoveries in SeGPX biochemistry and molecular biology in relation to its protective function are reviewed here in comparison to other selenoproteins. The mechanisms of selenium incorporation into selenoproteins is also described. In the future, antioxidant enzymes such as selenium-dependent glutathione peroxidase may find a use in biotechnology for the protection of enzymes and cellular membranes. While it is recognised that in cost terms the use of enzyme mimics of SeGPX may be more advantageous, large-scale in-vitro production of the selenoproteins is now possible and their value can be readily assessed.