Cognitive function, with survival and response on brain imaging, is increasingly regarded as an important outcome measure in patients with brain tumours. This measure provides us with information on a patient's clinical situation and adverse treatment effects. Radiotherapy has been regarded as the main cause of cognitive decline in these patients, because children with brain tumours can develop intellectual deterioration caused by radiotherapy. In long-term surviving patients, radiotherapy may indeed lead to cognitive deficits, or even dementia. Recent studies, however, have made clear that focal radiotherapy in patients with glioma is not the main reason for cognitive deficits. The tumour itself and other medical treatments contribute largely to the cognitive deficits. Cognitive function is now also recognised as an independent prognostic factor in the survival of glioma patients. Additionally, cognitive deterioration can be the first indicator of progressive disease after treatment.