Germ cells are the precursors of the sperm and oocytes and hence are critical for survival of the species. In mammals they are specified during fetal life, migrate to the developing gonads and then undergo a critical period during which they are instructed, by the soma, to adopt the appropriate sexual fate. In a fetal ovary, germ cells enter meiosis and commit to oogenesis, whereas in a fetal testis, they avoid entry into meiosis and instead undergo mitotic arrest and mature towards spermatogenesis. Here we discuss what we know so far about the regulation of sex-specific differentiation of germ cells, considering extrinsic molecular cues produced by somatic cells as well as critical intrinsic changes within the germ cells. This review focuses almost exclusively on our understanding of these events in the mouse model.

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