The electroencephalogram (EEG) plays an important diagnostic role in epilepsy and provides supporting evidence of a seizure disorder as well as assisting with classification of seizures and epilepsy syndromes. Emerging evidence suggests that the EEG may also provide useful prognostic information regarding seizure recurrence after a single unprovoked attack and following antiepileptic drug withdrawal. Continuous EEG video telemetry monitoring has an established role in the diagnosis of non-epileptic pseudo-seizures and in localizing the seizure focus for epilepsy surgery. Newer tools such as EEG mapping and magneto-encephalogram, although still investigational, appear potentially useful for defining the seizure focus in epilepsy. This review examines the traditional concepts of clinical EEG in the light of newly available data.