Ovarian senescence occurs gradually during the fourth and fifth decades of life, leading to menopause at an average age of about 51 years. This senescence results in a changing hormonal milieu, with decreases in the levels of estrogens and androgens. Similar changes may be induced by surgical menopause (bilateral oophorectomy) or ovarian failure resulting from cancer treatment. The declining levels of estrogens and androgens affect many tissues of the body and can produce a variety of signs and symptoms, including vasomotor symptoms, decreased bone density, changes in mood and energy, loss of pubic hair and changes in the genital tissues, and effects on sexual function. Accurate measurement of testosterone levels in postmenopausal women requires methods that are validated in the lower ranges of testosterone level observed in this population.