Cardiac surgery is responsible for profound modification in body water distribution, electrolyte plasma concentration, and acid–base balance. Maintaining homeostasis must take into account the type of surgery, the alterations due to anesthesia, the effects of cardiopulmonary bypass, the patient’s comorbidities, and their own response to surgical stress. The ideal approach to perioperative fluid management is still debated in all clinical contexts and in cardiac surgery patients in particular, since a load of fluid is generally needed because of cardiopulmonary bypass priming. The debate involves the kind of fluid to use (crystalloids vs colloids; colloid vs colloid; balanced vs unbalanced solutions) and the amount of fluid to administer (liberal, restrictive, Goal Direct Therapy). In this debate, economic interests have influenced literature production and results, making it more difficult for the interpretation of many results and complicating the clinical application of scientific findings in routine practice. Electrolytes are always modified after cardiac surgery. With respect to the past, the benefits of their administration (in particular, calcium) have been discussed in the literature. In this chapter, the basis of fluid and electrolyte management in cardiac surgery patients is explained, through understanding physiology and pathophysiology and considering with critical approach literature evidence.