In birds, sex determination occurs at fertilisation by the inheritance of sex chromosomes. The analysis of avian sex chromosomes and sex-determining genes has important implications for both wild and domesticated birds. This allows the sexing of fledgling or immature birds, and mature birds that are not sexually dimorphic, and provides critical information for the breeding of rare or endangered species. Second, by genetically manipulating key sex-determining genes, the sex of commercially produced birds can potentially be altered. This is of value in the poultry industry, for example, where females are required for laying, whereas males are required for meat production. Hence, all birds can now be sexed at the molecular level, enhancing studies on avian reproductive biology, ecology, breeding and conservation.