TOBEC measurements are based on the premise that a conductive object will interact with an electromagnetic field and result in the dissipation of a small quantity of the field's energy. TOBEC instruments provide a measure of this energy loss, which for a given conductor varies according to its size. The conductive component of the body is that part that contains water with dissolved electrolytes, i.e., essentially the FFM. Fat, by definition, is anhydrous and does not contribute to a TOBEC measurement. To interpret TOBEC readings in terms of the quantity of FFM in the body requires the use of a calibration equation that is generated by measuring the FFM of a reference population using an alternative technique and relating this to each individual's TOBEC value. Because no alternative method exists to estimate the FFM of an infant, a calibration equation is used that was generated from measurements of young miniature piglets. The body composition of infants thus derived is consistent with our general understanding of the changes in fat and FFM in human infants in early life. Whether TOBEC can be used as described in premature or older children has yet to be determined. Variations in the composition of the FFM do not seem to compromise the accuracy of the technique. TOBEC measurements are precise, rapid, noninvasive, and safe and cause the subject no discomfort. These features not only make TOBEC a useful tool for the nutritional assessment of a pediatric patient but they also may be a useful diagnostic tool for the clinician.