In the past several years, considerable interest has developed in the study of total body bioimpedance analysis (TBBIA) and body composition in healthy subjects. This simple and noninvasive technique uses derived and regression equations to validate data comparative to the gold standards of total body water determination and body composition. However, this approach has not proved to be of value in the critically ill patient with distorted body composition under the effects of multiple drugs and interventions. Notwithstanding, in recent years several authors have demonstrated the usefulness of this technique, apart from derived equations, relying solely in the judicious interpretation of the primary impedance parameters: resistance (R, ohms [omega], reactance (Xc, omega) and the derived phase angle (phi, degrees). The principles of bioelectrical impedance postulate that R is the opposition of total body water and electrolytes to the flow of an alternating current of low amplitude and high frequency, typically 800 microA and 50 kHz, and Xc is the capacitance produced by tissue interface and cell membranes. The phase shift is quantified geometrically as the angular transformation of the ratio Xc/R and is designated as phi. Altered cellular membrane function is a common feature of the septic patient and it has been systematically associated with a significant low Xc and phi. On the other hand, the fluid balance can be precisely monitored by the changes in R, and the serial measurements of the relation Xc/R (or the phi) has been utilized to discriminate between normal subjects and patients, and between septic and nonseptic critically ill patient. Some authors have demonstrated that this relation Xc/R is highly correlated with mortality and could be used for the staging of critically ill patients throughout their stay in the ICU. The role of TBBIA as a simple and noninvasive technique, and its implications for the management of critically ill patients are presented and discussed. Areas for future investigations, with single and multiple frequency, have the potential to clarify many aspects of this emergent technology at the bedside.