Summary Fluid therapy is a key component of resuscitation of critically ill patients. However, inadvertent administration of intravenous fluids can have deleterious effects on the patient outcome. Thus, the ability to identify patients who would respond to fluid administration by increasing stroke volume and hence cardiac output is of vital importance. This article attempts to define ‘fluid challenge’ and ‘fluid responsiveness‘ and also looks at the advantages and limitations of currently used strategies. The recent increase of research interest in this field reflects the evidence that early fluid optimisation of critically ill patients improves outcome. This concept has subsequently been extended into the peri-operative setting. A brief summary of the latest research in these fields is given.