Estrogens are responsible for the development of the secondary female sex characteristics. Together with gestagens they control all the important female reproductive processes. The biologically most active estrogen is 17β‑estradiol. This is a steroid hormone having a molecular weight of 272 daltons. Estrogens are produced primarily in the ovary (follicle, corpus luteum), but small quantities are also formed in the testes and in the adrenal cortex. During pregnancy, estrogens are mainly formed in the placenta. About 98 % of estradiol is bound to transport proteins (SHBG = sex hormone binding globulin). Estrogen secretion is biphasic during the menstrual cycle. The determination of estradiol is utilized clinically in the elucidation of fertility disorders in the hypothalamus‑pituitary‑gonad axis, gynecomastia, estrogen‑producing ovarian and testicular tumours and in hyperplasia of the adrenal cortex. Further clinical indications are the monitoring of fertility therapy and determining the time of ovulation within the framework of in vitro fertilization (IVF).