Abstract: Intravenous maintenance fluid therapy aims to replace daily urinary and insensible losses for ill children in whom adequate enteric administration of fluids is contraindicated or infeasible. The traditional determination of fluid volumes and composition dates back to Holliday and Segar’s seminal article from 1957, which describes the relationship between weight, energy expenditure, and physiologic losses in healthy children. Combined with estimates of daily electrolyte requirements, this information supports the use of the hypotonic maintenance fluids that were widely used in pediatric medicine. However, using hypotonic intravenous fluids in a contemporary hospitalized patient who may have complex physiologic derangements, less caloric expenditure, decreased urinary output, and elevated antidiuretic hormone levels is often not optimal; evidence over the last 2 decades shows that it may lead to an increased incidence of hyponatremia. In this review, we present the evidence for using isotonic rather than hypotonic fluids as intravenous maintenance fluid.