Publisher Summary This chapter discusses the luteinizing hormone releasing hormone (LHRH) agonists and antagonists and their pharmacological activities and clinical applications. A number of LHRH agonists have entered clinical trial. Potent antagonists of LHRH show good in vivo activity. A few LHRH antagonists have recently been reported to be active by the oral route. Because LHRH analogues are poorly absorbed by the oral route, considerable effort has been expended to devise suitable formulations for clinical trials. LHRH agonists can stimulate gonadal function if given at low dose as a series of pulses. The combination of LHRH agonists with pure anti-androgens has also been studied in prostate cancer and preliminary results suggest that survival may be improved. The clinical utility of LHRH analogues in other forms of cancer remains to be determined. After treatment with LHRH analogues, responses have also occurred in post-menopausal women with breast cancer. The gonadal suppressing action of LHRH agonists as also been used to treat successfully precocious puberty in both boys and girls; preliminary results suggest that it has advantages over other available therapies. There has been widespread interest in the use of LHRH analogues as contraceptive agents and numerous studies have been conducted in primates and man. As many highly potent and safe LHRH agonists are available, there is room for improvement in the potency of LHRH antagonists and ideally in the identification of the orally active agents.