Fluid therapy, rehydration, is a treatment to maintain the body’s fluid balance by replenishing fluids or controlling fluid intake by various means. It is commonly administered orally or through a gastric tube as well as intravenously, and fluid therapy is not used if it can be done without it, unless the condition requires it or the doctor recommends it. The amount of rehydration fluid needs to be determined according to the amount of dehydration and the type of rehydration fluid according to the nature of the dehydration. The principles of rehydration are: first fast, then slow, first thick, then light, first salt, then sugar, and rehydration with potassium when seen in urine. The prime time for rehydration is within 24 hours of the first day. If symptoms such as severe dehydration, hemorrhagic shock and severe acidosis occur, the amount of fluid lost by the body needs to be replenished, and the principle is to replenish how much is lost.