The discovery of estrogen receptor-β (ERβ) in 1996 stimulated great interest in the physiological roles and molecular mechanisms of ERβ action. We now know that ERβ plays a major role in mediating estrogen action in several tissues and organ systems, including the ovary, cardiovascular system, brain, and the immune system, and that ERβ and ERα generally play distinct physiological roles in the body. Although significant progress has been made toward understanding the molecular mechanisms of ERβ action, particularly in vitro, there remains a large gap in our understanding of the mechanisms by which ERβ elicits its biological functions in a true physiological context.