Bone loss occurs when the cellular events of bone formation are quantitatively larger than bone formation. This manuscript discusses the measurement of bone loss, occurrence in the population, risk factors and consequences of bone loss. Recent developments in bone mass measurement and biomarkers have improved our ability to assess bone loss. This process is a normal concomitant of ageing. There are a number of other risk factors, including sex hormone deficiency, physical inactivity, calcium/vitamin D deficiency, inflammatory arthritis, corticosteroids, smoking and alcohol. The major consequence of bone loss in our ageing society is fracture.