Glucose control in childhood type 1 diabetes is difficult and often characterized by significant glucose variability, including periods of prolonged hyperglycemia and intermittent episodes of hypoglycemia that can be severe. The brain of the developing child is thought to be more susceptible to metabolic insults because of its relatively high demand for glucose to fuel neuronal growth and differentiation. In this review we consider the impact of glucose variability, especially when associated with recurrent hypoglycemia, on long-term cognitive function in childhood type 1 diabetes. At present, this indicates a subtle effect of type 1 diabetes per se on a number of cognitive modalities. Within the population of children with type 1 diabetes, a history of severe hypoglycemia also appears to have an additional negative effect on cognitive function. However, interpretation of the literature is difficult in that the human studies draw largely from cross-sectional observational epidemiology while more basic work has used models that do not translate well into human disease. Moreover, it is likely to be many years before we will be able to clearly document the effects of recurrent hypoglycemia or chronic hyperglycemia on cognitive function. In the meantime, it seems appropriate to advocate that minimizing glucose variability when achieving glycemic targets should be the therapeutic goal of clinicians involved in the management of childhood type 1 diabetes.