Diabetes mellitus is the most common metabolic disease and its incidence is increasing. New classifications have been recently proposed based upon a better knowledge of pathogenesis, mechanisms of glucose transport and insulin resistance. The perioperative care of diabetic patients is less dependent on blood-glucose control, which needs to be exceptionally tight, and is usually easily obtained thanks to pharmacological improvements, human insulins and analogues, technical progress with blood-glucose monitoring at the bedside and infusion with constant flow rates. More important is the influence of end-organ pathology, often clinically silent, which must be carefully assessed during the preoperative evaluation. The organ impairments concern especially the heart, but also all those organs that were modified by abnormal glycosylated proteins. The pre-existing pathology has many consequences on anaesthesia management; the anaesthetic technique depends essentially on their existence. A better long-term control of diabetes both for Type 1 and Type 2 by insulins, and the new oral anti-diabetic drugs reduce the incidence of the end-organ pathology and the risk linked to organ failures in the perioperative period.