n males, estrogens exert pleiotropic effects by acting on several tissue and organs, including the male reproductive system. The action of estrogens is manifest from prenatal life during which the exposure to estrogen excess might influence the development of some structures of the male reproductive tract. Male fertility is under the control of estrogens, especially in rodents. The loss of function of estrogen receptor alpha and/or of the aromatase enzyme leads to infertility in mice. In men, estrogens are able to exert their actions at several levels through the reproductive tract and on several different reproductive cells. However, the regulation of human male reproduction is more complex and the role of estrogens is less clear compared to mice. During fetal and perinatal life, estrogen acts on the central nervous system by modulating the development of some areas within the brain that are committed to controlling male sexual behavior in terms of setting gender identity, sexual orientation development and the evolution of normal adult male sexual behavior. This organizational, central effect of estrogens is of particular significance in other species (especially rodents and rams), being probably less important in men where psychosocial factors become more determining. Other relevant, non-reproductive physiological events depend on estrogen in men and they involve bone maturation and mineralization as well as metabolic functions. In this chapter we provide an update of estrogen\u2019s role in male reproductive function by reviewing the physiological actions of estrogen on male reproduction and the pathophysiology related to estrogen deficiency and estrogen excess. Phenotypes associated with estrogen deficiency and excess in rodents and in man have shed new light on the mechanisms involved in male reproduction, challenging the perception of the predominant importance of androgens in men. It is now clear that the imbalance between estrogen and androgen in men might affect male reproductive function even in presence of normal circulating androgens. Some uncertainties still remain, especially regarding the impact of abnormal serum estrogen levels on male health, particularly due to the fact that estrogen is not routinely measured in men in clinical practice. Advancements in methods to precisely measure estrogens in men, together with a reduction of their costs, should provide better evidence on this issue and inform clinical practice. New basic and clinical research is required to improve our knowledge on the role of estrogen in male reproductive function and men\u2019s health in general. For complete coverage of all related areas of Endocrinology, please see our online FREE web-book, www.endotext.org
