Bioelectrical impedance (BI) relies on the conduction of a low-voltage alternating current through the body. Lean tissue and fluids containing electrolytes conduct the current and cell membranes serve as capacitors and account for capacitive resistance. Fat and bone are poor conductors. Measurement of the voltage drop of the applied current yields resistance (R) and reactance (Xc). R and Xc are used with height, weight, age, and gender in a number of multiple regression relationships to predict body composition compartments such as fat-free mass, lean body mass, extracellular mass, and body cell mass. The technique has been compared with and validated against traditional measures of body composition analysis. In clinical practice, BI has been used to monitor fluid status in burn and dialysis patients, assess changes of body cell mass with nutritional repletion, and predict pharmacokinetic parameters and dose of theophylline and aminoglycoside antibiotics. BI is a noninvasive, safe, rapid, and reproducible technique with exciting potential in clinical practice.