Since its introduction nearly ten years ago, dual-energy x-ray absorptiometry (DXA) has become the single most widely used technique for performing bone densitometry studies. One reason for its popularity is the ability of DXA systems to measure bone mineral density (BMD) in the spine and proximal femur, the two most common sites for osteoporotic fractures. Other advantages of DXA include the exceptionally low radiation dose to patients, short scan times, high resolution images, good precision and inherent stability of calibration. For these reasons DXA scans are widely used to diagnose osteoporosis, assist making decisions in treatment, and as a follow-up response to therapy. Another important application has been the use of DXA in many clinical trials of new treatments for osteoporosis. Since the first generation pencil beam DXA systems became available, the most significant technical innovation has been the introduction of fan beam systems with shorter scan times, increased patient throughput, and improved image quality. New clinical applications include the measurement of lateral spine and total body BMD, body composition, and vertebral morphometry. Despite these advances, posteroanterior (PA) spine and proximal femur scans remain the most widely used application because of their utility in treatment decisions and monitoring response to therapy.