Aromatase converts irreversibly androgens into estrogens and is present in the endoplasmic reticulum of various cells of mammalian testes ; at least in rodents, all testicular cells except peritubular cells express aromatase. In testis, high affinity estrogen receptors, ERalpha and/or ERbeta, together with a membrane rapid effect recently described, mediate the effects of estrogens. From the experimental models, in vitro studies and data collected in patients, it is now demonstrated that estrogens play an important role in the testis of vertebrates. At least it is supported by the widespread distribution of estrogen receptors in the testicular cells and the simultaneous presence of a biologically active aromatase in all germ cells (especially in meiotic and post-meiotic stages). Thus the role of estrogens (intracrine, autocrine and / or paracrine) in spermatogenesis (proliferation, apoptosis, survival and maturation) and more generally, in male reproduction is now evidenced, and much more complex than previously predicted.