The intention of this paper is to critically review the current state of knowledge of the role of the brain in the syndrome of hypoglycaemia unawareness. Both the role of the brain in the detection of hypoglycaemia and initiation of the counterregulatory responses and the function of the cerebral cortex during acute hypoglycaemia are considered. The evidence for and against the brain as the primary site of mammalian hypoglycaemia sensing and the mechanisms whereby such sensing may occur and change in hypoglycaemia unawareness are discussed. Current evidence supports a major role for the central nervous system in hypoglycaemia sensing and there is increasing understanding of the mechanisms of counterregulatory failure and cognitive dysfunction in hypoglycaemia unawareness. More needs to be done to expand this understanding and translate it into therapeutic strategies to defend against severe hypoglycaemia in diabetes therapy.