In birds, sex determination occurs at fertilisation by the inheritance of sex chromosomes. This review summarises our current understanding of sex determination in birds, with emphasis on the molecular genetics of male versus female development during embryonic life. Recent studies in the Chicken (Gallus gallus domesticus) have revealed some remarkable features of avian sex determination, such as the finding that sex appears to be determined autonomously within cells throughout the body, and the demonstration that the key, sex-linked gene, DMRT1, is required for testis formation and hence male development. However, despite these recent advances, the mechanism of avian sex determination is still not entirely clear. Understanding sex determination in birds has important implications for the conservation of threatened species, and for the global poultry industry.