Although use of the bioelectrical impedance method for the indirect assessment of human body composition is growing, awareness of the theoretical and practical bases of the method is limited. I discuss the relations between impedance variables and biological indexes, describe the biophysical models for the translation of impedance variables into in vivo body-composition indexes, and summarize the influences of physical and biological factors that affect the validity of the bioelectrical impedance method. The practical advantages of the bioelectrical impedance method necessitate concerted research efforts to establish the most appropriate applications of the method in research and clinical medicine.