In the fetus, the predominant energy supply is glucose transported across the placenta from the mother. As pregnancy progresses, the amount of glucose transported increases, with glycogen and fat stores being laid down, principally in the third trimester. In the well-term baby, there is hormonal and metabolic adaptation in the perinatal period to ensure adequate fuel supply to the brain and other vital organs after delivery, but in the preterm infant, abnormalities of glucose homeostasis are common. After initial hypoglycaemia, due to limited glycogen and fat stores, preterm babies often become hyperglycaemic because of a combination of insulin resistance and relative insulin deficiency. Hyperglycaemia is associated with increased morbidity and mortality in preterm infants, but what should be considered optimal glucose control, and how best to achieve it, has yet to be defined in these infants.