Patients with refractory focal epilepsy are thoroughly evaluated to identify an area of cortex that, if removed or disconnected, will lead to seizure freedom. Clinical semiology, neuroimaging, and scalp electroencephalogram provide an approximation of this area, whereas intracranial recording may permit a more precise localization and investigation of a selected cortical area. Intraoperative electrocorticography delineates the irritative zone, and subdural electrode implantation also permits cortical stimulation of eloquent areas. Intraoperative electrocorticography rarely captures spontaneous seizures and may be influenced by the effect of anesthetic drugs, and the correlation between complete resection of the irritative zone and postsurgical seizure outcome is unclear. Extraoperative monitoring is often superior to intraoperative electrocorticography but may also be associated with more risk of adverse events. Further development of ultrahigh-density electrode arrays is providing novel insights into the role of microseizures and high-frequency oscillations on ictogenesis and epileptogenesis.