Patients in the intensive care unit (ICU) easily have large amounts of extracellular fluids, such as edema or ascites, because of cardiovascular instability under septic conditions and also have high risk of malnutrition while staying in the ICU. Traditional nutritional assessment parameters like body mass index have a limitation in ICU patients due to muscle atrophy and decrease of lean body mass. Bioimpedence analyses (BIA) can be used to assess body composition and are useful in performance of nutritional assessments in ICU patients. BIA can simply and noninvasively estimate body composition (total body water, extracellular water, intracellular water, body cell mass, and free fat mass etc.) by sending a weak electric current through the body. In particular, phase angle (PhA, phase difference between the voltage applied to the impedance and the current driven through it), one of the parameters of BIA, is related to cell membrane integrity or cell size. Low PhA can possibly imply malnutrition and PhA has been reported as a useful indicator of clinical outcomes or prognosis of severe patients. Additional study with clinical application of BIA in ICU patients is needed in order to confirm the usefulness of BIA.