This chapter provides a substantial review of information about human sexual reproduction. Sex differentiation in fetal development is explained, including genotypic and phenotypic normalities and differences. The roles of genes (especially the SRY region of Y), developing gonads, and hormones in fetal development are discussed, as well as the importance of maternal hormone levels, for physical, structural, and neural fetal development. Prenatal hormones are particularly important for the development of gender identity, with modern research suggesting in utero hormone exposure affects brain development and later childhood and adulthood gender-typed behaviors and qualities, including sexual orientation. Various anomalies may occur that affect sex differentiation, including genetic (de la Chapelle Syndrome, Swyer Syndrome) and hormonal disorders (CAH, AIS). Puberty (development of secondary sex characteristics, menstruation, spermatogenesis), fertilization, and pregnancy are explained, including the dangers of pregnancy to women and the role of mother’s nutrition and teratogens on fetal development.