Physical aspects favour subdural registrations: as the solid angle under which a source is seen from a nearby electrode, the potential is large; there is no distortion by high impedence between source and electrode, nor by perpendicular short circuits; the frequency response is quasi-linear. Technical aspects and implantation strategy are outlined. Interictal and ictal ECoG patterns of diagnostical relevance and activation procedures are discussed. Advantages of subdural electrodes are: They provide detailed information on the epileptogenic zone, data on prognosis, are free of artefacts, allow electrical stimulation to determine eloquent cortex areas; by mathematical analysis methods the localizing power is increased. Disadvantages are: closed fields can be missed (recordable only by depth electrodes), in comparison to EEG, costs are considerably higher. Risks are practically absent with strip electrodes. In about half of the investigated patients, subdural electrodes were necessary to minimize the area of resection on a rational basis.