Colloid solutions contain oncotic or colloid osmotic molecules, which are mainly distributed to the intravascular space (IVS). Crystalloid solutions are distributed over the entire extracellular space, especially in the interstitial space (ISS) as it is the largest component of the extracellular space. They are primarily used to maintain a patient’s requirements of water and sodium and are frequently referred to as maintenance fluids. The general principles of fluid replacement and the principles for resuscitation of body spaces with various solutions have been explained in this chapter. Total fluid administered over the perioperative period must provide daily maintenance of water, electrolytes and fluid to replace losses of extracellular fluid, blood or other abnormal losses. This chapter also includes perioperative fluid therapy which includes assessment of maintenance fluid comprising of normal maintenance fluid, correction of pre-existing deficits, abnormal losses resulting from the underlying pathology and calculation of replacement fluid therapy.