In 1949, Dr. Juhn Atsushi Wada, a Japanese neurologist treating patients with schizophrenic personality disorder, discovered that internal carotid artery injection of isopentobarbital to anesthetize one hemisphere allowed identification of the patient’s dominant hemisphere for language function. This method was subsequently used by Dr. Wada and Dr. Rasmussen to lateralize the dominant language hemisphere during the preoperative evaluation of patients with epilepsy, with excellent clinical results. Since then, the Intracarotid Amobarbital Procedure (IAP) has been routinely used in the preoperative evaluation of refractory epilepsy to localize the dominant hemisphere, and the IAP was later called the Wada trial after its inventor, Dr. Wada. It is now also used in the preoperative assessment of the dominant hemisphere in pars plana and deep temporal lobe tumors.