Sex determination in vertebrates is accomplished by gonad differentiation in the embryo, which unleashes a cascade of hormones that control sexual phenotype. The pathway by which gonad (testis or ovary) is differentiated is highly conserved in all vertebrates, but the trigger (genetic or environmental) that initiates the whole process may be quite different between lineages. Among species with genetic sex determination, the trigger gene, and its mode of action as a male- or female-dominant, or a dosage sensitive, is known in only a few species. Patterns are starting to emerge that hint at ways in which an autosomal gene may acquire ways of regulating genes at the head of the gonad differentiating pathway, usurp the sex determining function and define new sex chromosomes. The raw material for new sex-determining genes may be genes in the sex differentiating pathway, related genes, or even genes with no known role in sex. The changes that make these genes sex determining can be as simple as a change in the timing or tissue of expression. Intriguingly, certain genes (such as DMRT1 and SOX3) seem to have been independently pressed into service in different ways in distantly related lineages. Developmental Dynamics 242:354–359, 2013. © 2012 Wiley Periodicals, Inc.