The administration of adequate fluid to the severely dehydrated patients takes precedence over all other treatment regardless of the underlying disease. Although the understanding of pathogenesis and the appropriate management of water, electrolytes, and acid-base disturbance are emphasized in medical education, many physicians are not very confident in treating the patients with those disturbances. The first thing that should be remembered in fluid therapy is how much of water and sodium is required to the patients-in other words, how much water with what concentration of sodium should be given to the patients. Water deficit can be estimated by assessing the severity of dehydration, while the sodium deficit from the types of dehydration, that is, the plasma sodium concentration of the patients. This review will summarize the principles of how to choose and give the adequate fluid according to the severity and the types of dehydration.