Osteoporosis is characterized by decreased bone mass and increased susceptibility to fractures. The clinical consequences of osteoporosis are fracture, most commonly seen at the wrist, the spine and the hip. The prevalence of osteoporotic fracture is extremely high, with almost 50 percent of 70-year-old women having had at least one osteoporotic fracture. The prevalence in men is lower but may constitute a larger problem than expected. Osteoporotic fracture is therefore a significant cause of morbidity and mortality and represents a major problem of health care. Bone is the site of substantial metabolic exchanges, with bone resorption and formation continuing throughout life. The turnover processes are carried out by osteoclasts and osteoblasts in specialized cellular units. After cessation of growth, the skeleton probably consolidates to reach the peak bone mass at the age of 35-40 years. The relatively slow subsequent age-related bone loss occurs in both men and women, but women are additionally exposed to accelerated bone losses after the menopause. Oestrogen deficiency is the dominating pathogenetic factor for osteoporosis in women and is the drug of choice for prevention and treatment. Today it is established that hormone replacement therapy (HRT), using estrogen alone or estrogen plus a progestogen, can prevent osteoporosis in postmenopausal women. The greatest benefit from HRT is obtained if instituted shortly after menopause. However, the literature contains clear evidence that HRT prevents the bone loss in all stages of the postmenopausal life. A recent study of hormone replacement therapy of women with symptomatic osteoporosis has furthermore demonstrated that the incidence of fractures was significantly reduced during therapy.