During the intraoperative and post-operative period, fluids are administered to cover the patient\'s maintenance requirement and replace losses due to surgery. The types of fluids used can be divided into three broad categories, blood and blood components, colloids and crystalloids. Crystalloids are fluids that contain water and electrolyte composition similar to extracellular fluid. These solutions are used primarily to provide a patient’s maintenance requirements of water and sodium and are frequently referred to as maintenance fluids. These fluids when administered intravenously, are able to pass through the capillary membrane. If deciding which fluid is to be administered for the different surgeries has been a little tricky until now, reading this chapter will make the choice crystal clear. The different crystalloids, their properties, their indications, contraindications have been explained here.