The inability to clear glucose adequately from the blood is a common feature of obesity. The frequency of impaired glucose tolerance has been reported to vary from 11.5% to 74% on obese children: the causes of this discrepancy in the literature are discussed. Carbohydrate intolerance occurs in obesity in spite of the basal and reactive hyperinsulinaemia. This condition is termed insulin resistance, which can develop in several ways. The different theories concerning the primary cause or causes of insulin resistance in obesity are reviewed.