The administration of intravenous fluids for resuscitation is the most common intervention in acute medicine. There is increasing evidence that the type of fluid may directly affect patient-centred outcomes. There is a lack of evidence that colloids confer clinical benefit over crystalloids and they may be associated with harm. Hydroxyethyl starch preparations are associated with increased mortality and use of renal replacement therapy in critically ill patients, particularly those with sepsis; albumin is associated with increased mortality in patients with severe traumatic brain injury. Crystalloids, such as saline or balanced salt solutions, are increasingly recommended as first-line resuscitation fluids for the majority of patients with hypovolaemia. There is emerging evidence that saline may be associated with adverse outcomes due to the development of hyperchloraemic metabolic acidosis, although the safety of balanced salt solutions has not been established. Fluid requirements vary over the course of critical illness. The excessive use of fluids during the resuscitative period is associated with increased cumulative fluid balance and adverse outcomes in critically ill patients. The selection of fluid depends on the clinical context in which it is administered and requires careful consideration of the dose and potential for toxicity. There is an urgent need to conduct further high-quality randomized controlled trials of currently available fluid therapy in patients with critical illness.