Estrogen is the central component in 6 of the 100 most widely prescribed medications in the United States today. This steroid has several therapeutic uses including contraceptive applications, treatment of menopausal symptoms, and the prevention of osteoporosis. A wide variety of estrogen replacement therapy (ERT) and estrogen plus progestational hormonal replacement therapy (HRT) preparations are available. In addition, there are an increasing number of products with estrogenic properties that are being promoted as alternatives to drugs containing estrogen, such as phytoestrogens and selective estrogen receptor modifiers (SERMs). This article reviews the physiology of estrogenic effects, estrogen metabolism, and the pharmacokinetics of marketed preparations.