The endometrium undergoes cellular and structural changes that are essential for its function. These changes are cyclical and controlled by the production of estrogen and progesterone by the ovaries. Drugs with estrogenic or progestogenic modes of action also lead to alterations in the ultrasonographic appearances of the endometrium. This chapter helps the sonographer to interpret the appearances of the endometrium in women. The Committee on Safety of Medicines (CSM) has advised that hormone replacement therapy (HRT) is beneficial for the treatment of menopausal symptoms. Although unscheduled bleeding on HRT requires investigation, the underlying malignancy risk is low. In postmenopausal bleeding, there is a cut off of 4 mm for double-layer endometrial thickness on Transvaginal ultrasound (TVS) but for women on HRT there is no agreed cut-off point. Hormonal contraceptives and intrauterine contraceptive devices (IUCD) have multiple effects leading to their contraceptive action.