The thresholds for the diagnosis of diabetes are currently based on the glycaemic levels above which diabetes-related microvascular complications mostly occur. The aetiological classification of diabetes was described by the World Health Organization (WHO) and also approved by the American Diabetes Association. The WHO has published several guidelines for the diagnosis of diabetes since 1965. The classification of type 2 diabetes is largely characterized by exclusion. For many years, gestational diabetes was defined a state of carbohydrate intolerance resulting in hyperglycaemia of variable severity, with onset or first recognition during pregnancy. The advances made in the past two decades in diagnostic and research methodologies for identifying pathophysiological components of various types of diabetes have provided significant clarity in the classification and diagnosis of diabetes. Nevertheless, the classification of diabetes continues to evolve as underlying genetic and other factors become identified with increasing precision.