The colloid/crystalloid controversy is largely an artificial construct. If the interstitial space is dry, use a crystalloid and if the intravascular space is dry, use a colloid. There has been a controversy over the best of fluids for resuscitation, i.e. crystalloids or colloids. Anaesthesiologists must, therefore, have understanding of the current consensus on crystalloids and colloids in the perioperative period. Colloids have large molecules which exert an oncotic pressure on the plasma membrane and are retained within the vascular system more effectively than crystalloids. Colloid solutions are generally administered in a volume equivalent to the volume of blood lost. The initial volume of distribution is equivalent to the plasma volume. Crystalloid solutions are also plasma substitutes that contain water and electrolytes. They are grouped as balanced, hypotonic or hypertonic salt solutions. Administered crystalloid is distributed in a ratio of 1: 3 in extracellular space, i.e. plasma: interstitial fluid. The current controversies about fluid and volume management which involve the amount of fluid replacement, the new consensus regarding the various formulae and intravenous fluids, controversy over crystalloids versus colloids have been dealt with in great detail in this chapter.