In women and men, an important proportion of estrogens and androgens are synthesized locally at their site of action in peripheral target tissues. This new field of endocrinology has been called intracrinology. In postmenopausal women, 100% of active sex steroids are synthesized in peripheral target tissues from inactive steroid precursors while, in adult men, approximately 50% of androgens are made locally in intracrine target tissues. The last and key step in the formation of all estrogens and androgens is catalyzed by members of the family of 17β-hydroxysteroid dehydrogenases (17β-HSDs) while different 17β-HSDs inactivate these steroids in the same cell where synthesis takes place. To date, seven human 17β-HSDs have been cloned, sequenced and characterized. The 17β-HSDs provide each cell with the means of precisely controlling the intracellular concentration of each sex steroid according to local needs.