Intravenous fluids are commonly prescribed but uncertainty remains about how to assess when fluids are required and how much to give, particularly in our multimorbid, polymedicated and ageing population. Furthermore, studies have noted that fluid resuscitation can be harmful even if clinical evidence of hypervolaemia is not present. Two recent guidelines have acknowledged a limited evidence base to guide fluid assessment. A recommended means to assess hypovolaemia includes assessment of fluid responsiveness. Fluid responsiveness is a rise in stroke volume following an increase in preload, achieved using a fluid challenge or a passive leg raise. However, the means of defining fluid responsiveness and its ability to identify patients who would benefit from fluid resuscitation is currently unclear. This review discusses the current guidelines about, and the evidence base for the provision of, intravenous fluids in the acutely unwell medical patient. It highlights how little evidence is available to guide medical practice.