In both children and adults, it has been well established that the precise localization of a seizure focus allows for the best possible resection and outcome. Long-term invasive monitoring with DE has been the most widely used modality to evaluate patients with intractable seizures of temporal lobe origin. The study of epilepsy in children, however, requires assessment of the cause of the seizures with a resultant decision regarding the optimal modality of study for the particular problem. Complex partial seizures of medial temporal lobe origin without clear concordance or lateralizing information are best studied using DE. With MR imaging anatomic localization, stereotactic insertion of DE can be performed accurately and relatively safely. The information obtained permits lateralization and focus localization if the lesion is within the medial structures. The ability of DE to study epilepsy outside of the medial temporal lobe, however, is suboptimal. Many centers have used surface electrodes to map temporal lobe foci successfully. SE and SGE have not been widely used in children. Subdural electrode arrays, however, are superior to DE in extratemporal epilepsy because the cortical surface contacts provide the best recordings of epileptiform activity from a wide range of extratemporal regions. The limitations of intraoperative ECoG in the awake child are well understood. It is rare that a child can undergo surgery under local anesthesia and cooperate to the extent necessary for definitive mapping of the seizure focus and eloquent areas. Thus, invasive long-term monitoring would seem ideal in the epileptic pediatric patient because it allows for a more relaxed pace in which to record events, and because of the indwelling nature of these electrodes, it is possible for the investigator to map eloquent areas of the brain before resection. This type of evaluation completely and accurately maps interictal and ictal activity, and through cortical stimulation or mapping of somatosensory areas by evoked potentials, eloquent areas of the brain, such as motor and speech regions, can be identified. This method is limited to older children because the young child often has inadequate cortical development to localize these areas definitively. One should note that in young children (under the age of 4 years), the absence of a response to standard cortical stimulation does not indicate nonfunctional cortex. Long-term intractable seizures and significant antiepileptic drug requirements may contribute to developmental and psychosocial deficiencies in the epileptic patient.(ABSTRACT TRUNCATED AT 400 WORDS)