Patients with disorders of sodium and water metabolism are seen frequently in Emergency Departments, but in most cases these disorders do not require treatment. Severe hypo- and hypernatremia are relatively uncommon but can result in severe neurologic sequelae or death. Appropriate therapy, based on an understanding of the physiology involved, can reduce the morbidity and mortality associated with these disorders. A systematic approach to the assessment and management of patients with hypo- and hypernatremia is presented.