Bone density scans using the technique of dual energy X-ray absorptiometry (DXA) are widely seen as having an essential role in the evaluation of patients at risk of osteoporosis. DXA scans of the spine and hip are the most useful because fractures at these sites result in the greatest impairment of quality of life. A hip DXA scan is the most reliable way of evaluating hip fracture risk, and the spine is the most sensitive site for monitoring response to treatment. Fundamental to the clinical role of DXA scanning are the findings of epidemiological studies that relate fracture risk to bone mineral density. The results of spine and hip DXA scans are interpreted using the World Health Organisation (WHO) definition of osteoporosis as a T-score less than -2.5. DXA scans are also used in longitudinal studies, both for research studies and for follow-up scans of individual patients. Although the former have an important role in the development of new treatments for osteoporosis, the latter are more controversial and are likely to be less important in the future. There is also interest in the use of smaller, less expensive DXA systems for scanning sites in the peripheral skeleton. However, in general, the results from these devices cannot be interpreted using the WHO definition of osteoporosis, and until there is a consensus over establishing an equivalent threshold, their use is premature.