Electrolyte and fluid imbalances are disorders frequently observed in critical care patients. In many instances patients are asymptomatic, but they may also present with neurological alterations, severe muscle weakness, nausea and vomiting or cardiovascular emergencies. Therefore, a pathophysiological understanding of these disorders is necessary for initiating an appropriate therapy. After a precise history-including drug prescriptions-has been obtained from the patient or his/her relatives, determination of the hydration status of the patient and measurement of acid-base status, plasma and urine osmolality and electrolytes are the first steps in the assessment of the disease. Once a diagnosis has been established, great attention has to be paid to the rate at which the disorder is corrected because this-if inappropriate-may cause more severe damage to the patient than the disease itself. This chapter addresses the initial diagnostic and therapeutic steps of the most common electrolyte emergencies.