Abstract The safe administration of i.v. fluids is one of the most significant advances in the care of critically ill patients this century. However, despite advances in the monitoring of cardiovascular variables, the question of what? when? and how much? remain areas of enormous controversy. Ironically as the choice of i.v. fluids becomes greater and the monitoring more sophisticated the controversy grows. This article will concentrate on the diagnosis of hypovolaemia, the consequences of hypovolaemia and the rational use of i.v. fluids, particularly colloids, to treat it.