Body composition (BC) assessment is indispensable to evaluate nutritional status and thus health, both at the population and individual level, and to assess the efficacy of primary and secondary preventive nutritional strategies. Changes in BC, including the regional distribution of body fat, largely occur during pubertal transition, with marked differences between genders. They may, however, also occur in the elderly, who experience significant changes in the ratio between body fat and muscle with aging. The development and implementation of more sophisticated techniques (e.g. BC assessment at the molecular and atomic levels) could provide a major contribution to determining BC at different levels. This review discusses the application of dual-energy X-ray absorptiometry (DXA) on BC determination, given that DXA has the potential to provide overall and regional assessment of BC in terms of fat, lean mass and bone. DXA is widely used in many clinical settings primarily diagnosis osteoporosis. This article describes the use of whole-body DXA in assessing BC in patients with chronic diseases (e.g. metabolic syndrome) as well as in different sport activities to evaluate the effects of exercise.