Osteoporosis has been defined as a systemic ­skeletal disease that is characterized by low bone mass and micro-architectural deterioration of bone tissue, with a consequent increase in bone fragility and susceptibility to fracture. The most frequent osteoporotic fractures are fractures of the hip, wrist, and spine, although most fractures in the elderly are probably at least partly related to osteoporosis. Osteoporosis is more common in women and, in this Primer, we focus on postmenopausal osteoporosis. Bone mineral density (BMD) is a key tool in identifying osteoporosis and its introduction into clinical practice 30 years ago was a major step forward. A working group of the WHO has defined osteoporosis operationally as a BMD that is 2.5 standard deviations below the peak bone mass for adult women, or lower. The number of standard deviations from average is defined by the T-score, and a T-score of ≤–2.5 is thus defined as osteoporosis. The limitation of this approach is that it increases a risk factor to the status of a diagnostic criterion and so ignores the other determinants of bone strength and fracture propensity. In the same way that we estimate the risk of cardiovascular disease from key risk factors, there has been a move to use BMD along with key risk factors for fracture (particularly age, weight, and prior fracture) to predict fracture in the next