As survival among children treated for cancer continues to improve, more attention is being focussed on the late effects of cancer treatment. In children treated for brain tumours, chronic neurocognitive effects are especially challenging. Deficits in cognitive development have been described most thoroughly among children treated for posterior-fossa tumours, specifically medulloblastomas and ependymomas, which account for about 30% of all newly diagnosed cases of brain tumours in children. Most children who have survived brain tumours have required surgical resection and focal or craniospinal radiotherapy (irradiation of the entire subarachnoid volume of the brain and spine), with or without systemic chemotherapy. Historically, intelligence quotient (IQ) scores have provided a benchmark against which to measure changes in cognitive development after treatment. Observed declines in IQ are most likely a result of failure to learn at a rate that is appropriate for the age of the child, rather than from a loss of previously acquired knowledge. The rate of IQ decline is associated with a several risk factors, including younger age at time of treatment, longer time since treatment, female sex, as well as clinical variables such as hydrocephalus, use of radiotherapy and radiotherapy dose, and the volume of the brain that received treatment. Loss of cerebral white matter and failure to develop white matter at a rate appropriate to the developmental stage of the child could partly account for changes in IQ score. Technical advances in radiotherapy hold promise for lowering the frequency of neurocognitive sequelae. Further efforts to limit neurocognitive sequelae have included design of clinical trials to test the effectiveness of cognitive, behavioural, and pharmacological interventions.