The human testis manufactures over one hundred million sperm cells daily. This prodigious task depends upon a highly coordinated process ensuring efficient cellular replication and a finely tuned differentiation program. Spermatogenesis is a cyclic process in which diploid spermatogonia differentiate into mature haploid spermatozoa. A pool of undifferentiated progenitor cells is held in reserve since most spermatogonia enter a stem cell renewal pathway by undergoing mitotic division (in a manner analogous to somatic cells). Only a minority of spermatogonia are committed to further differentiation by dividing mitotically to give rise to diploid spermatocytes. Then meiosis takes place. Rather than splitting into daughter cells, primary spermatocytes undergo two meiotic divisions that result in the production of haploid round spermatids. These are sculpted into the shape of mature spermatozoa by the process of spermiogenesis, which involves dramatic biochemical and morphological restructuring of the germ cell (Figure 1). In mice, the entire developmental process takes 35 days.