Osteoporosis is a skeletal disorder characterized by low bone mass and microarchitectural deterioration of bone tissue, leading to an increase in bone fragility and susceptibility to fracture. From the age of 50 years, 50% of women and 20% of men will be concerned by fracture during their remaining lifetime. An operational definition of osteoporosis has also been defined, based on a value for bone mineral density (BMD) 2.5 standard deviations or more below the young adult mean (Kanis, Osteoporosis. 4:368–381, 1994). The most widely validated technique for the quantitative assessment of BMD is dual-energy X-ray absorptiometry. Bone loss is due to an imbalance between bone resorption by osteoclasts and bone formation by osteoblasts, which are part of the bone remodeling process. Several lifestyle, metabolic, and immunological factors contribute to accelerated bone loss in addition to ageing and genetic factors. Many pathological conditions (e.g., endocrine diseases, chronic inflammatory diseases) or treatments (e.g., corticosteroids) also interfere with bone metabolism and promote osteoporosis. Several pharmacological agents are available to lower fracture risk by reducing bone resorption or/and by stimulating bone formation.