Hyperglycaemia is a frequent phenomenon in critically-ill patients, associated with increased morbidity and mortality. Hyperglycaemia results in cellular glucose overload and toxic adverse effects of glycolysis and oxidative phosphorylation, especially in tissues with insulin-independent glucose uptake, and acute hyperglycaemia can exert a variety of negative effects. It is the main side effect of intensive insulin therapy. Both severe and moderate hypoglycaemia are independent risk factors of mortality in critically-ill patients. Prolonged hypoglycaemia induces neuronal damage, but may also have adverse cardiovascular effects. Several risk factors predispose critically-ill patients to hypoglycaemic events. Rapid glucose fluctuations may induce oxidative stress and lead to vascular damage. Glucose complexity is a marker of endogenous glucose regulation. Association between hyperglycaemia and outcome is weaker in diabetic critically-ill patients than in non-diabetic patients. Pre-admission glucose control in diabetic critically-ill patients plays a role in the response to glucose control and mortality.