Introduction: Assessing the hydration status of critically ill patients has been a difficult task over the decades. Determining how much fluid overload a patient has often helped in choosing a therapy. Methods such as bioelectrical impedance have been approached as a useful tool for this purpose. Objective: This study proposes to verify, through research in the literature, what is the real importance of the clinical use of bioelectrical impedance in the diagnosis of fluid overload in critically ill patients hospitalized in intensive care units. Methods: bibliographic search in the main scientific information databases: Scielo, PubMed, Cochrane, and Lilacs from January 2000 to July 2018. The selected languages were Spanish, Portuguese, and English. The keywords used were bioelectrical impedance, hydration, Intensive Care Unit, Intensive Care, bioelectrical impedance analysis, fluid balance, hydration overload. Results and Conclusion: The analysis of fluid overload in critically ill patients can be performed using multifrequency bioelectrical impedance. It is a useful tool in the diagnosis as well as in the quantification of water overload and, therefore, a corroborative method for clinical decision-making.