It is estimated that as many as 10% of couples worldwide suffer from infertility or reduced fertility, and that in approximately half of these cases this results from defective spermatogenesis. In 60% of these infertile men, the failure to produce mature germ cells (i.e. azoospermia) or the formation of low numbers of sperm (i.e. oligozoospermia) can be ascribed a genetic aetiology. Few of the loci associated with male infertility have been mapped in humans; however, several genetic models of defective germ-cell development and differentiation have been described in mice. The schematic in Figure 1 summarizes the main features of spermatogenesis in humans.