Aromatase p450 (p450arom) catalyzes the formation of estrogen in several human tissues under the control of alternatively used promoters. Each promoter is regulated by a distinct signaling pathway in a tissue- and hormone-specific manner. This article reviews these regulatory mechanisms in the ovary, adipose tissue, placenta, bone, brain, and various malignant cells. Moreover, the physiological and pathologic impacts of local estrogen biosynthesis in contrast to circulating estrogen or estrogen-responsive human tissues are discussed.