Catabolism of fat-free mass affects both the quality of life and survival of patients. Because of variations in fluid status during acute illness, changes in body weight are difficult to evaluate and interpret during treatment. Nutritional assessment should therefore evaluate fat-free and fat mass changes during metabolic stress and catabolism. We have chosen to discuss bioelectrical impedance analysis, including the various bioelectrical impedance analysis techniques (i.e. multi-frequency and bioimpedance spectroscopy), as an easy, non-invasive, portable bedside technique that is operator-independent for evaluating fat-free and fat mass compartments and their changes during treatment. Clinical examples of the determination of fat-free mass in healthy and ill individuals are also presented.