The usefulness of electroencephalography (EEG) as an aid to diagnosis of seizure disorders is established, but its role as a guide to monitoring treatment is much less certain. For those patients with classical absences and 3-s spike wave activity there is a very close correlation between control of clinically detected seizures and EEG events. In some, but not all, patients with other seizure disorders there is a positive correlation between numbers of seizures and amount of interictal epileptiform activity (IEA). Intravenous benzodiazepines and phenytoin result in both acute seizure control and suppression of IEA. For seizures other than absences, and antiepileptic drugs (AEDs) given in the medium and long term, there is generally not a clear relationship between control of seizures and IEA. In studies of children whose epilepsy is in remission, persistent IEA has been associated with a higher risk of seizure relapse should AEDs be discontinued, but in adults the relevance of persistent IEA appears to be much less certain. Benzodiazepines and barbiturates result in increased fast activity. All AEDs may result in slowing of the dominant rhythm and increased slow activity. Carbamazepine, in particular, is often associated with apparent deterioration of background activity, even in the face of clinical improvement. Further studies are necessary to determine the mechanisms and significance of AED-induced changes in EEG background activity.