Diabetes mellitus is a complex metabolic disorder, caused by defects in insulin action and/or insulin production and is defined as afasting hyperglycaemia of >126 mg/dl, with normoglycaemia being ≥70 and ≤ 110 mg/dl. There are two main types of diabetes. Type 1 diabetes (around 10% of cases) is an autoimmune disease, usually of early onset, in which pancreatic islet beta cells that secrete insulin are destroyed. Type 2 diabetes (around 85% of cases) is characterised principally by insulin resistance and impaired insulin secretion. Heredity and obesity are major risk factors for Type 2 diabetes. Diabetes is associated with potentially life-threatening microvascular and macrovascular complications caused by elevated serum glucose levels. Treatment of diabetes aims at restoring glycaemic control. In Type 1 diabetes, this can be achieved by injecting insulin. Oral hypoglycaemic medications that stimulate insulin secretion and/or modify glucose metabolism can be used as a first-line treatment in Type 2 diabetes mellitus. However, insulin is usually necessary in later phases of the disease. Lifestyle changes, such as diet and exercise, are also important. Glycaemic control can be measured by fasting blood glucose levels and also by glycosylated haemoglobin levels. The latter measure gives an indication of glycaemic control over a period of three months, and a reduction in glycosylated haemoglobin is the most appropriate treatment goal in the management of diabetes.