Hyperglycemia recognized during hospitalization for acute coronary syndromes (ACS) is common. It is a powerful predictor of worse prognosis in patients both with and without previously known diabetes mellitus. Hyperglycemia during ACS is caused by an inflammatory and adrenergic response to ischemic stress, when catecholamines are released and glycogenolysis induced. The link between hyperglycemia and prognosis in ACS patients is multi-factorial. Hyperglycemia exerts detrimental effects on left ventricular and platelet function and it also activates other systemic pathological processes that contribute to cellular and tissue injury such as increasing oxidative stress and worsening endothelial function. Glucose management strategies in ACS may improve outcome in patients with hyperglycemia, by reducing inflammatory and clotting mediators, by improving endothelial function and fibrinolysis and by reducing infarct size. Most of available oral anti-diabetic drugs are contraindicated for the increased risk of hypoglycemia. Thus, insulin is the preferred agent for glycemic control in ACS and conversion from intravenous to subcutaneous therapy commonly occurs when the critical acute phase of ACS resolves. Pharmacodynamics of insulin allows it to be adaptable to the changing physiology of the ACS patient, is easily titrated and has no dosage threshold. Nevertheless, findings concerning the effect on ACS-related mortality of the control of glucose levels by intravenous infusion of insulin have been conflicting and intervention trials are needed to optimize the definition of hyperglycemia and to establish appropriate modalities and goals of glucose lowering treatment. In particular, the clinical benefit of an aggressive treatment with insulin is yet unproved.