Hyperglycaemia is a common feature of the critically ill and has been associated with increased mortality. Maintaining normoglycaemia with insulin therapy improves survival and reduces morbidity in prolonged critically ill patients in both surgical and medical intensive care units, as shown by two large randomized controlled studies. Prevention of cellular glucose toxicity by strict glycaemic control appears to play a predominant role, but also other metabolic and non-metabolic effects of insulin appear to contribute to the clinical benefits.