Book excerpt: This book presents the current state of knowledge on the origin and differentiation of cell lines involved in the development of the vertebrate male and female gonads with particular emphasis on the mouse. It also discusses the processes leading to the testis- and ovary-specific structures and functions. 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.