Hypovolaemia is the most common cause of circulatory failure in children. Treatment consists of volume suppletion with a crystalloid or colloid solution; which agent is the best in children is not clear. This evidence-based practice guideline formulates recommendations as to which fluid should be used for volume suppletion in critically-ill neonates and children up to the age of 18 years with hypovolaemia. Before the guideline development first-choice fluid for volume resuscitation was in 50% a colloid and in 50% a crystalloid solution for both neonatologists and paediatric intensivists. The neonatologists used human albumin as a priority, and the paeditric intensivists predominantly used a synthetic colloid. The guideline was developed on the basis of a comprehensive search and analysis of the literature according to the principles of evidence-based guideline development. The recommendations were formulated by a committee based on evidence from the literature and, when evidence from the literature was insufficient, on consensus after discussion in the committee. Since colloids are much more expensive than crystalloids and can give an anaphylactic reaction, their added value over crystalloids must be proven. In sick neonates and children, insufficient clinical trials have been done to reach the conclusion that colloids are more effective than crystalloids in hypovolaemia. A number of meta-analyses in adults revealed excess mortality in the group treated with albumin, but one recent, large, randomised study showed no difference in mortality. No added value could be demonstrated for the administration of synthetic colloids. On the basis of data from the literature and considerations regarding the applicability of evidence in adults to children and neonates, the side effects of resuscitation fluids, pathophysiology and costs, the first-choice fluid for neonates and children with hypovolaemia is isotonic saline. Albumin should not be used for the treatment of hypovolaemia. The volume to be administered and the infusion rate depend on the severity of the hypovolaemia and should be determined on an individual basis.