The field of human body composition research by necessity is based upon assumptions related to tissue chemical content. The 2-compartment model, which requires only 1 measurement in addition to body weight, is the cornerstone of current research in this field. Yet the assumptions upon which 2-compartment models are based were developed on a limited scale and their validity under specified conditions is questionable. Recent developments now allow quantification of previously unmeasured somatic constituents. The capacity to extend our models to 4 or more components is now at hand. Not only will this allow us to vastly expand our validation of two 2-compartment approaches, but our ability to explore new and important physiological questions is within reach.