EEG abnormalities in migraine have been reported by a number of authors during the last 50 years. Prevalences vary considerably in the older literature. A number of unspecific rhythms related to drowsiness or hyperventilation have probably been counted as "abnormal", and the reported numbers of definitely abnormal EEG rhythms have been consistently low. In a few controlled and blinded studies, however, slight excess of various EEG rhythms has been found in migraine patients. Similar prevalences of interictal EEG abnormalities have generally been found in patients with classic and common migraine, but the diagnostic classification may not have been precise enough in some studies. During visual aura, either slow waves, depression of background activity amplitude or normal EEG have been reported. The most definitely abnormal EEGs with unilateral or bilateral delta activity have been recorded during attacks of hemiplegic migraine, and during attacks of migraine with disturbed consciousness. The relationship between migraine and epilepsy has still not been adequately clarified. The connection seems to exist in several small entities (e.g. migraine-like headache as an epileptic manifestation, epileptic seizures triggered by epileptic attacks, and possibly in epilepsies with occipital spike waves), but it is seemingly not "fundamental". Newer methods, i.e. EEG frequency analysis and topographic brain mapping, are promising tools in this field. So far, mostly small studies have been published with somewhat inconsistent results. A pattern of increased alpha rhythm variability (and/or asymmetry) in the headache-free phase seems to emerge, however. Significant asymmetry of alpha and theta during headache has been reported in one topographic brain mapping study. Magnetoencephalographic studies of migraine patients have demonstrated slow wave-shifts (similar to those observed in animals with spreading depression). The EEG patterns observed in migraine patients seem to suggest a possible physiological connection between sleep, hyperventilation and migraine. The study of such relationship may shed new light on migraine pathophysiology.