Glutathione peroxidase (GSH-Px) was described as a selenoprotein in 1973 and, since then, a great body of information has been accumulated to validate its important role in the antioxidant defence network in all animals, including poultry. The GSH-Px family includes at least eight members, and four of them (GSH-Px1, GSH-Px2, GSH-Px3 and GSH-Px4) are shown to be selenoproteins in animals. They are characterised by species- and tissue-specificity in their expression and activity. An optimal Se status in tissues/body is key for maximum expression of GSH-Px and therefore, in avian research GSH-Px activity is widely used as a biomarker for determining Se status and requirements. On the other hand, GSH-Px is an inducible enzyme and its activity depends on the level of stress and can be used as an index of antioxidant defences. In poultry production two forms of Se-dependent GSH-Px (GSH-Px1 and GSH-Px4) have received most attention. The aim of this paper is to review GSH-Px properties and functions in relation to poultry biology with special emphasis to its role in chicken adaptation to various stress conditions. Recent advances in selenoprotein identification and characterisation in relation to poultry Se status, dietary sources of Se and stress conditions can shed light on the roles of GSH-Px in avian biology.