Bone mineral density (BMD) testing is a noninvasive measurement to diagnose osteoporosis or low bone density, predict fracture risk, and monitor changes in bone density over time. The "gold-standard" technology for diagnosis and monitoring is dual-energy x-ray absorptiometry of the spine, hip, or forearm. Fracture risk can be predicted using a variety of technologies at many skeletal sites. BMD is usually reported as T-score, the standard deviation variance of the patient's BMD compared with a normal young-adult reference population. In untreated postmenopausal women, there is a strong correlation between T-score and fracture risk, with fracture risk increasing approximately two-fold for every standard deviation decrease in bone density. BMD in postmenopausal women is classified as normal, osteopenia, or osteoporosis according to criteria established by the World Health Organization. Standardized methodologies are being developed to establish intervention thresholds for pharmacologic therapy based on T-score combined with clinical risk factors for fracture.