This article is an introduction to dual X-ray absorptiometry (DXA), the most widely used method today for diagnosis of osteoporosis. DXA can be used to assess projective bone mineral density at the lumbar spine, the proximal hip, and the whole body as well as the skeletal periphery at the forearm, the hand, and the heel. The prominent area of application of DXA is the diagnosis and monitoring of osteoporosis and its treatment. Because of its high accuracy, precision, and ability to predict osteoporotic fracture as well as its relatively low cost, DXA has prevailed over alternative methods. This article discusses the underlying X-ray physics and technological aspects, acquisition protocols, quality characteristics, and sources of error and their relevance. It also describes the various skeletal regions accessible to measurement, details on precision, nominal results, usability to predict fracture risk, and results of influential clinical trials.