Cessation of ovarian estrogen secretion is the key event during the climacteric. An enzyme termed aromatase in a number of human tissues and cells, including ovarian granulosa cells, the placental syncytiotrophoblast, adipose and skin fibroblasts, bone, and the brain, catalyzes the conversion of C19 steroids to estrogens. Aromatase expression in adipose tissue and possibly the skin primarily accounts for the extraglandular (peripheral) formation of estrogen and increases as a function of body weight and advancing age. Sufficient circulating levels of the biologically active estrogen, estradiol, can be produced as a result of extraglandular aromatization of androstenedione to estrone, which is subsequently reduced to estradiol in peripheral tissues, to cause uterine bleeding and endometrial hyperplasia and cancer in obese anovulatory or postmenopausal women. Extraglandular aromatase expression in adipose tissue and skin (via increasing circulating levels of estradiol) and bone (via increasing local estrogen concentrations) is of paramount importance in slowing the rate of postmenopausal bone loss. Moreover, excessive or inappropriate aromatase expression was demonstrated in adipose fibroblasts surrounding a breast carcinoma, endometriosis-derived stromal cells, and stromal cells in endometrial cancer and gave rise to increased local estrogen concentrations in these tissues. Whether systemically delivered or locally produced, elevated estrogen levels promote the growth of these steroid-responsive tissues. Finally, local estrogen biosynthesis by aromatase activity in the brain may be important in the regulation of various cognitive and hypothalamic functions. The regulation of aromatase expression in human cells via alternatively used promoters, which can be activated or inhibited by various hormones, increases the complexity of estrogen biosynthesis in the human body. Aromatase expression is under the control of the classically located proximal promoter II in the ovary and a far distal promoter I.1 (40 kb upstream of the translation initiation site) in the placenta. In adipose tissue, two other promoters (I.4 and I.3) located between I.1 and II are used in addition to the ovarian-type promoter II. To add a further twist, promoter use in adipose fibroblasts switches between promoters II/I.3 and I.4 upon treatment of these cells with prostaglandin E2 (PGE2) versus glucocorticoids plus cytokines. Moreover, the presence of a carcinoma in breast adipose tissue causes a switch of promoter use from I.4 to II/I.3. Molecular and cellular mechanisms responsible for estrogen formation and their physiologic and clinical relevance will be reviewed in this article.