The evaluation of the total body composition by discriminating its different components is a very useful tool in clinical practice. Bioelectrical impedance analysis (BIA) has become a good tool for this assessment. The technique relies upon electrical properties of the body that were described since 1871 and has the possibility of estimating extracellular fluid/intracellular fluid volume ratio. Changes in this ratio underlie many critical clinical conditions and diseases. In this chapter, its principles, assumptions, and clinical uses in its various modalities are described.