Since glucose is a vital fuel for the brain, hypoglycaemia demands prompt recognition, effective treatment, and accurate identification of the cause. However, pointing the cause, especially in children, can be and often is difficult. Neonatal hypoglycaemia is of a particular importance for paediatricians, since it is, when prolonged or recurrent, a preventable cause of mental retardation and permanent neurological damage. Neonatal hypoglycaemia can be transient or persistent and both of these groups can be further subdivided into two subgroups, where the aetiology of hypoglycaemia is primarily due to decreased hepatic glucose production or to increased peripheral glucose utilization. This article describes first part of causes of neonatal hypoglycaemia.