The current views of sex determination in birds are considered mostly with the example of Gallus gallus domesticus, the species best studied in this respect. Data on the appearance of primordial germ cells, their migration to the primordial gonads, the role of hormonal factors in the regulation of sex differentiation, the sex chromosomes, putative genetic mechanisms of sex determination, and a possible contribution of dosage compensation are described. The review discusses the two best-grounded hypotheses on the roles of the Z and W chromosomes in sex determination.