The human reproductive system begins to mature in fetal life and is remarkably active during the first months after birth. During childhood, the reproductive system becomes quiescent until subsequent reactivation begins pubertal development. Puberty is heralded by an increase in pulsatile secretion of gonadotropin-releasing hormone (GnRH) from the hypothalamus followed by an augmented pituitary response to GnRH and escalating secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH). This sequence, which results in gonadal maturation (see Chap. 91) and increasing levels of sex steroids, leads to development of the secondary sexual characteristics, acceleration of growth, and ultimate fertility (see Chap. 16 and Chap. 91). Although the mechanism of this reactivation has been largely elucidated, factors that determine the timing of pubertal onset remain elusive.