Summary Fluid challenges are used to improve cardiac output and oxygen delivery. This is done in the presence of hypovolaemia. Hypovolaemia is generally diagnosed on static haemodynamic parameters, such as Central Venous Pressure, Pulmonary Capillary Wedge Pressure etc. Only about half of patients administered fluid in this manner, have benefitted. Recently interest has been directed at functional haemodynamic parameters. These are based on the relationship between mechanical ventilation and venous return due to the cyclical changes in intra thoracic pressure. These cause variation in the stroke volume, systolic blood pressure and pulse pressure during the respiratory cycle. There are factors which can affect the accuracy and the interpretation of these parameters. Spontaneous respiration, the tidal volumes used to ventilate, PEEP, lung and chest compliance, heart rhythm, right ventricular function are these factors. Passive leg raising can be used in the presence of these or in doubt. This article attempts to set out, how to determine whether a fluid challenge will improve the cardiac output, and also to identify the problems in arriving at that decision.