Epileptiform EEG discharges are not confined to people with epilepsy, and their frequency is only weakly related to severity. A fundamental principle of EEG practice is, therefore, to avoid overinterpretation of epileptiform activity. Epileptiform discharges not accompanied by obvious clinical events are generally regarded as subclinical or interictal. However, in many patients sensitive methods of observation, notably continuous psychological testing, show brief episodes of impaired cognitive function during such discharges. This phenomenon of transitory cognitive impairment (TCI) is found in about 50% of patients who show discharges during testing. TCI is not simple inattention. The effects are material and site specific: lateralised discharges are associated with deficits of functions mediated by the hemisphere in which the discharges occur. Conversely, specific tasks can activate or suppress focal discharges over the brain regions that mediate the cognitive activity in question. TCI clearly contributes to the cognitive problems of some people with epilepsy and may cause deficits that pass unrecognised. TCI is demonstrable in many cases of benign partial epilepsy of childhood, a disorder once thought to have no adverse psychological effects. TCI can contribute to abnormalities of psychological test profiles and interferes with daily tasks, such as reading and driving. In children it may be associated with behavioural disorders. An important practical issue is whether TCI materially impairs psychosocial function and, if so, whether drug treatment is desirable or effective. Uncontrolled reports and two preliminary randomised controlled trials of antiepileptic treatment of TCI have suggested that suppression of discharges is associated with significant improvement in psychosocial function.