The calculation of body composition from measures of body density is the classic method with which most persons are familiar. Traditionally it involves submerging the subject in a water tank and measuring the subject’s weight while under water. More recently, devices have become available to measure body volume using small changes in air pressure in an enclosed chamber. This laboratory has facilities for and has carried out thousands of underwater weighing measures for both clinical and research purposes. The available system includes two components, the underwater weighing tank with digital scale and the residual lung volume apparatus. The subject’s weight underwater, their weight on land, and residual lung volume are used to calculate body density. The procedure requires about 30 minutes and is safe, non-invasive, and accurate relative to other reference methods. The body density measurement is used to derive the fraction of body weight as fat and remainder, termed fat-free mass. The underwater weighing system is unique in having the capacity to evaluate subjects who are very tall or obese. The present system was used to evaluate professional basketball players and morbidly obese subjects weighing several hundred pounds. A recent alternative to measuring body density is referred to as air displacement plethysmography. Subjects enter a sealed container and sit quietly breathing through a mouthpiece. The system is designed to use changes in pressure and volume within the chamber to derive an estimate of body volume and body density. Results are comparable to underwater weighing, although development for very small subjects, notably children, is ongoing. Subjects up to about 250 pounds can fit comfortably within the system. Underwater Weighing/Air Plethysmography Applications and Background History Over one century ago investigators sought a means of non-destructively establishing the oil content of fish. A novel means was devised whereby the fish specific gravity was established and oil estimated using a two-component model. The model qualitatively assumed that one component was oil with a low specific gravity and the second component was remaining tissue with a higher specific gravity. In effect, the specific gravity became a measure of the fish oil content. By the mid 1940’s the technique had been refined and extended by Albert Behnke and his colleagues. Behnke devised an underwater weighing system that included correction of specific gravity for residual air trapped in the lungs. Behnke and his colleagues also proposed a quantitative two-compartment model consisting of fat and fat-free mass, each with known and assumed constant densities. Siri and other workers later refined these densities such that fat is now assumed to have a density of 0.9 g/cm3 and fat-free mass a density of 1.1 g/cm3 at body temperature. Assuming these two known and constant density’s and given the subjects measured density by underwater weighing, one can then compute the percentage of body weight as fat. The hydrodensitometry or underwater weighing method served as a reference technique for at least four decades against which other methods were compared. Application The modern technique involves a watertight tank typically with an electronic scale either positioned above the tank or submerged on the floor of the tank. The subject exhales, expelling air, submerges, and underwater body weight is recorded. Density is then calculated from the subjects’ weight in air and their weight under water following corrections for water temperature. The subject’s residual lung volume is also measured either during the underwater weighing procedure or following the underwater weighing after they have emerged from the tank. Underwater weighing systems are relatively easy to construct and are inexpensive. Accordingly, many systems have been built throughout the world including in developing countries. The information provided has contributed significantly to our understanding of human body composition in health and disease. The two-component model assumes that body weight consists of fat and fat-free mass. The density of fat is well established in both humans and animals and is approximately 0.9 g/cm3 . Only minimal variation is recognized within and between species in the density of fat. This is because fat, or specifically lipid, is almost entirely triglycerides. On the other hand, fat-free mass is a heterogeneous compartment consisting of at least four major components including water, protein, glycogen, and minerals. The density of these components varies from a low of 0.994 g/cm3 for water to a high of 3.04 g/cm3 for minerals. The assumed density of 1.1 g/cm3 is based on observations made in a limited number of human cadavers suggesting relatively stable proportions of water, protein, glycogen and minerals. To the extent that these proportions change in any individual subject will introduce corresponding errors in the assumed density of fat-free mass. A number of studies suggest that the density of fat-free mass is relatively stable across age and sex groups, although some variation is recognized at the extremes of age and in patients who have underlying medical and surgical conditions. Additionally, there may exist race differences in the density of fat-free mass as well as variation among special groups such as body builders or other types of athletic participants. Thus, while underwater weighing and the two-compartment model served as a reference technique for several decades, newer approaches without these various assumptions are now replacing hydrodensitometry as the clinical reference method. The importance of the underwater weighing method is that measurement systems are relatively inexpensive to construct, simple to operate, and the procedure is safe for patients varying widely in age and body weight. The method’s disadvantage is that some subjects have concerns about water submersion and thus either decline to participate or have technically inadequate measurements. Underwater weighing is also a technique that by necessity is stationary and cannot be moved around for field applications. A reasonable amount of technician skill is required to perform the method that under optimal conditions has a small technical error of about 0.001 g/cm3 . The residual lung volume measurement can also be difficult from some patients and adds to the measurement error adds to the method’s total error. The main concept of hydrodensitometry, that the body consists of two compartments of known and stable density, is very robust and as mentioned has served investigators well for several decades. This has led to a search for alternative means of quantifying body density in humans. A new approach to estimating body volume and body density is referred to as air plethysmography. The air plethysmography technique is based on classical gas laws. Small volume changes are produced within a two-chambered plethysmograph and the corresponding pressure change measured. The subject’s body volume is determined by subtraction of the empty chamber volume. Additional corrections are made for body volume based on body surface area and thoracic gas volume. The adjusted value for body volume is used in estimating body density and fat mass is then calculated from body weight and the classical Siri equation (Siri, 1956). The available commercial system, BODPOD® (Life Measurements Instruments, Concord, CA), has been widely validated in adults and children. The system is adequately portable for use in isolated areas for phenotyping assuming electrical power is available and ambient conditions such as temperature and humidity are adequately controlled and are relatively stable. The currently available system is optimized for adults and smaller systems for use in infants and children are in development. While many underwater weighing systems can accommodate extremely obese subjects, BODPOD’s size is finite and not all obese subjects fit within the fixed chamber volume.