The reproductive system consists of a series of feedback loops involving the higher centers, the hypothalamus, the pituitary, and the gonads. The factors involved in physiologic restraint of the hypothalamic pituitary gonadal axis until the time of puberty are complex. The pattern (frequency and amplitude) of GnRH signal is important in regulating pituitary LH and FSH secretion. This signal can be amplified and modulated at the pituitary level at least in part by the sex steroids. Hypothalamic hypogonadism can be considered a disorder of the hypothalamic GnRH pulse generator that results in deficient or dysrhythmic GnRH release. The mechanisms underlying the abnormal GnRH release in acquired, functional disorders such as anorexia nervosa and amenorrhea of joggers remain controversial. Evaluation of patients with hypothalamic hypogonadism involves exclusion of hyperprolactinemia, space-occupying lesions, and other systemic disorders. The pulsatile administration of GnRH for induction of fertility represents a major advance in the treatment of these patients.