Knowledge of estrogen kinetics is essential to clinicians providing gynecologic care. During the past decade, concepts involving estrogen kinetics have been key elements in the design of oral contraceptives, in the development of menopausal replacement therapy, in the tailoring of reproductive hormone support for transplanted embryos, and in hormone therapy of malignancies. In addition to affecting the female reproductive system, estrogens modulate insulin resistance, bone maintenance, muscle physiology, the cardiovascular system, and neoplastic processes. This chapter examines the process by which estrogens are delivered to the circulation and the intermediate metabolic processes that modulate their clearance and presentation to the microcirculation of target organs. It then integrates these principles into the clinical pharmacology of therapeutic estrogens