Inappropriate intravenous fluid therapy is a significant cause of patient morbidity and mortality and may result from either incorrect volume (too much or too little) or incorrect type of fluid. Fluid overload has no precise definition, but complications usually arise in the context of pre-existing cardiorespiratory disease and severe acute illness. Insufficient fluid administration is readily identified by signs and symptoms of inadequate circulation and decreased organ perfusion. Administration of the wrong type of fluid results in derangement of serum sodium concentration, which, if severe enough, leads to changes in cell volume and function, and may result in serious neurological injury. In patients whose condition is uncomplicated, we recommend a restrictive approach to perioperative intravenous fluid replacement, with initial avoidance of hypotonic fluids, and regular measurement of serum concentration of electrolytes, especially sodium.