The individual chapters review the origin and differentiation of the somatic cells of the genital ridges; the formation and migration of primordial germ cells in mouse and man; the gonadal supporting cell lineage and mammalian sex determination; differentiation of Sertoli and granulosa cells; mesonephric cell migration into the gonads and vascularization; origin and differentiation of androgen-producing cells in the gonads; germ cell commitment to the oogenic versus spermatogenic pathway and the role of retinoic acid; ovarian folliculogenesis; control of oocyte growth and development by intercellular communication within the follicular niche; biology of the Sertoli cell in the fetal, pubertal and adult mammalian testis; mechanisms regulating spermatogonial differentiation; stem cells in mammalian gonads; the role of microRNAs in cell differentiation during gonad development; human sex development and its disorders; as well as methods for the study of gonadal development.