Diuretics are important drugs in the treatment of fluid overload and other related conditions in infants and children. Their rational use depends on adequate knowledge of how they work and how they interact with other drugs and the changing physiology caused by normal growth and development and by various disease states. It is in the best interest of the patient for the physician prescribing these drugs to have appropriate knowledge of their mode of action. It is clear that with the exception of a few drugs the amount of appropriate data available on the use of these drugs in children is limited at best. As newer drugs become available, the physician caring for such children must be sure that there is some clear indication for its use in preference to those with which we have more experience.