Osteoporosis is a disease of reduced skeletal mass and microarchitectural deterioration of the skeleton, resulting in an increased risk of fractures. With the aging of the population, the considerable financial and personal costs of osteoporosis will increase. The loss of ovarian hormone production in menopause is the major risk factor for osteoporosis, causing increased skeletal resorption and relatively decreased bone formation. Estrogen administration protects from the loss of bone mass and reduces the incidence of vertebral and hip fractures, although administration must be continuous for the beneficial effects to remain. These effects occur regardless of the route of administration or the concomitant use of a progestin. Alternatives to estrogen are currently in development, including calcitonin, bisphosphonates, and tissue-specific estrogens.