GnRH and its analogues have led to exciting new avenues of therapy in virtually every subspecialty of internal medicine as well as in gynecology, pediatrics, and urology. Since their discovery in 1971, it has been demonstrated that GnRH and its analogues enable medical professionals to influence the hypothalamic-pituitary-gonadal axis in two distinct classes of therapeutic applications. The first provides natural sequence GnRH in a pulsatile fashion via portable infusion pumps to mimic the normal physiology of hypothalamic GnRH secretion and restores reproductive potential to infertile men and women with disorders of endogenous GnRH secretion. The second mode uses long-acting GnRH agonists administered in a depot delivery to produce a paradoxical desensitization of pituitary gonadotropin secretion which, in turn, results in a complete ablation of the reproductive axis. This biochemical castration induced by GnRH agonist administration is a safe, effective, complete, and reversible method of removing the overlay of gonadal steroids from a variety of diseases which they are known to exacerbate. These diseases include endometriosis and uterine fibroids in women, prostate cancer in men, and precocious puberty in both sexes. This review examines the physiologic and pharmacologic principles underlying the advances produced by these agents, the mechanism of action of GnRH and its analogues at the cellular level, and the individual therapeutic applications to which these analogues have been applied. Because virtually every subspecialty of medicine will be touched by the GnRH analogues, this review provides an overview and background of their use.