Measurement of body composition is an important part of nutritional assessment. The low FFM associated with malnutrition has been associated with numerous infectious and noninfectious complications, increasing length of stay, morbidity and mortality. DXA and TBK are reference method for determination of FFM and BCM, but, as these methods are expensive and require extensive technique of the operator, we have focused especially on BIA, an easy, quick, safe and reliable bedside method to measure body composition. BIA formulas to routinely assess FFM and appendicular skeletal muscle mass have been developed. Normative values of total body composition have been established, according to age and gender. Longitudinal and cross-sectional studies allowed an insight on the impact of physical activity and environment on body composition. With regard to clinics, we have studied the impact of body composition, determined by BIA, on length of hospital stay and shown that a low FFM and FM index were associated with an increased length of stay. This demonstrates that prevention of FFM loss, whether through nutritional support or drugs, may improve clinical outcome and decrease hospital costs. Finally, we have shown two case reports where sequential FFM measurements guided nutritional and medical therapy. Future studies should focus on the relationship between body composition and outcome in various types of patients. They should also try to determine the characteristics of nutritional support (amount of calories, type of macronutrient intakes, timing of nutritional support), potentially associated with anabolic drugs or physical activity, necessary to limit FFM loss, in order to improve clinical outcome.