Because of the spatial limitations of scalp electroencephalographic recordings and the lack of reproducible results using functional MR imaging techniques to map various eloquent areas of the cortex, the accurate mapping of the extent of epileptogenicity and function currently are best accomplished through invasive electrocorticographic recordings and direct cortical stimulation, respectively. The purpose of this article is to give an overview of the presurgical diagnostic tools used in patients with suspected malformations caused by abnormalities of cortical development with special emphasis on subdural electrodes. The indications and the role of invasive subdural electrode evaluation are discussed.