A crucial stage in pregnancy is the implantation of the embryo into the endometrium. The endometrium, which is situated in the lining of the uterus, is a complex structure consisting of a single epithelial layer overlying a mesenchymal stroma. Cyclical growth of the endometrium under the influence of ovarian steroids results in the establishment of a receptive endometrium. If implantation fails, or is deliberately prevented by the use of fertility regulation, the endometrium is shed by the process of menstruation. This unique system of cyclic tissue regeneration is dependent on the cyclical growth and regression of the blood vessels that supply the endometrium. Little attention has been paid to this crucial aspect of human reproduction but it now appears that many disorders that reduce the quality of life of modern women, such as heavy menstruation, endometriosis, breakthrough bleeding and infertility, might have their basis in disorders of the endometrial vasculature.