Circulating insulin crosses the blood-brain barrier (BBB) into the central nervous system (CNS). There are many insulin receptors in various areas of the brain; they are expressed by both astrocytes and neurons. The two main insulin actions in the brain are (a) control of food intake and (b) effect on cognitive functions. In obesity there is a relative insulin deficiency in the CNS despite increased circulating levels. Insulin plays an important role in cognitive functions as demonstrated by the intranasal administration of insulin bypassing the liver. Brain insulin decreases with aging and may be related to the decrease in cognitive functions, as has also been reported in Alzheimer's disease. Certain brain tumours over-express insulin receptors. Whether the larger insulin analogues pass the BBB is as yet not known.