Premenstrual syndrome (PMS) is a psychological and somatic disorder of unknown aetiology. The symptoms of PMS regularly occur during the luteal phase of the menstrual cycle and resolve by the end of menstruation. The severe and predominantly psychological form of PMS is called 'premenstrual dysphoric disorder'. PMS results from ovulation and appears to be caused by the progesterone produced following ovulation in women who have enhanced progesterone sensitivity. This enhanced sensitivity may be due to neurotransmitter dysfunction. Treatment is aimed at suppressing ovulation or reducing progesterone sensitivity. This chapter will describe the role of hormones and hormonal treatments in PMS.