: Determining body contents such as body water volume and body cell mass have significant uses in health and disease. Accumulation of extracellular water is particularly difficult to monitor using classical methods. Bioelectrical impedance analysis (BIA) is a simple, rapid, and noninvasive method, based on the principle that the flow of altering electrical current through a particular tissue differs depending on the content of water and electrolytes. It is thus able to measure body composition, including total body and extracellular water. Although bioimpedance holds up quite well compared to the gold standard that is dual-energy X-ray, it has certain limitations in critically ill patients. Specifically, it cannot distinguish between intravascular and interstitial volume in the extracellular compartment, and as it employs equations based on population measurement, compositions can diverge significantly with severe overhydration or in the morbidly obese. Bioelectrical vector analysis (BIVA) does not use the calculations and is part of the measurements in newer multifrequency bioimpedance devices. There is growing evidence of the adverse effect of overhydration in critically ill patients and bioimpedance can be used to monitor hydration, but there is no information on how to use this method for bedside monitoring in practice. In this review we present a practical approach to Phase angle and BIA/BIVA interpretations for monitoring hydration status and rapid loss of skeletal muscle mass and their clinical use, on a cohort of critical COVID patients under artificial lung ventilation.