Bioimpedance analysis is method for evaluating body composition using a small harmless electrical current passed across body tissues. The Laboratory has pioneered the development of new BIA systems over the past decade. Many commercial BIA systems have also been evaluated in the Laboratory that houses about 10 state-of-the art systems for carrying out BIA in subjects of all ages. Some simpler systems are based on a single frequency while other rely on multiple frequencies and more complex methods for deriving body fat, fat-free mass, skeletal muscle, body water, and water distribution. Bioimpedance Analysis in Body Composition: Background and Applications History Bioimpedance analysis (BIA) methods were first conceived and evaluated for use in the study of human physiology during the 1930’s by Burger and Barnett. Modern BIA methods are all based on a similar principle: resistance to an applied alternating electrical current is a function of tissue composition. With technological improvements, by the mid 1980’s new and practical BIA methods were introduced for estimating total-body water, fat-free mass, and total-body fat. These methods proliferated, and by 1994 the National Institute of Health organized a watershed conference in which the clinical and research applicability of BIA methods were reviewed in detail. All tissues conduct an electrical current to an extent dependent upon the biological characteristics of each specific tissue. Tissues with long cylindrical cells and a high fluid and electrolyte content such as skeletal muscle impose relatively little resistance to an applied electrical current. In contrast, tissues with globular cells and a low water content such as adipose tissue pose a relatively high resistance to the conductivity of an alternating electrical current. This varying specific tissue resistivity allows empirical development of body composition prediction models. While advanced methods permit actual imaging of tissues, current BIA methods usually are developed by first setting the body component of interest as the dependant variable in prediction models. These dependant variables often include total-body water measured by the reference method of isotope dilution and fat-free mass as measured by DXA or underwater weighing. Measured resistance, or closely related impedance, is then set as the predictor variable in regression models. The electrical measurements are usually adjusted first for path length, typically as length2 or height2. Other predictor variables can also be inserted such as age, sex, race, and even selected anthropometric measurements. The developed prediction model is then cross validated prior to application. Application Bioimpedance analysis methods vary widely in the nature of the applied electrical current and the selected measurement pathway. Two system types are available, single frequency and multiple frequency. Single frequency systems are typically based on a 50 kHz alternating current. These systems provide a measure of impedance or resistance at a frequency of 50 kHz and some may also provide corresponding estimates of reactance and phase angle. The electrical properties are then used in body composition prediction models as previously described. Electrodes can vary from typical gel electrodes that must be applied first to stainless steel electrodes that maintain pressure contact with the subject. More advanced systems are based on multiple frequencies. These applied electrical frequencies can vary from as little as 1 kHz up to 1 MHz. Multiple frequency systems are applied when measuring fluid compartments and they offer little advantage to single frequency systems for estimation of fat-free mass and total-body fat (73). The measured electrical circuit can also vary, ranging from an isolated region of a single leg to multiple limbs and the whole body (76). The most common approach is the “half body” pathway extending from one arm to the corresponding leg. An essential feature of BIA methods is that measurements are taken according to standardized conditions as suggested by the manufacturer or developer. Time of day, subject position, room temperature, level of prior physical activity, and meal ingestion are all variables that should be considered (77,78). When measurements are taken carefully, body composition estimates are usually well correlated with those provided by underwater weighing, DXA, and other reference methods (79). Consideration should also be given to the quality of the selected instrument and the appropriateness of the instrument’s calibration equations for the subjects under evaluation. As BIA systems are relatively inexpensive to purchase and simple to operate, they have wide applicability in clinical and field settings. Results will be acceptable if properly applied and calibrated systems are used in evaluating appropriately selected subjects. BIA estimates are appropriate for long-term patient monitoring and in phenotyping groups of subjects for genetic studies. BIA body composition estimates are not generally reliable for evaluating short-term changes in body fluid or fat mass, although newer multiple frequency systems offer promise as a means of tracking short term fluid balance (80).