Hemispherectomy is a neurosurgical procedure in which a cerebral hemisphere (half of the upper brain, or cerebrum) is removed or disconnected that is used to treat a variety of refractory or drug-resistant seizure disorders (epilepsy). Refractory or drug-resistant epilepsy is defined as seizures that fail to be controlled using 2 or more appropriate anti-seizure medications. About one in three patients with epilepsy have drug-resistant epilepsy and of those, about half of them have focal epilepsy that can potentially be treated with epilepsy surgery. In drug-resistant epilepsy where all or most seizures arise from one hemisphere, hemispherectomy is a highly effective procedure producing seizure freedom in about 80-90% of patients. In addition to controlling seizures and as a result of that, improved development and cognition is also very frequently achieved after hemispherectomy. Most patients who qualify for hemispherectomy already have neurological deficits such as hemibody weakness or loss of vision in one visual field that is opposite to the side of the affected hemisphere. This means that performing hemispherectomy does not add deficits to a new hemisphere in these patients. However, detailed testing is always performed prior to epilepsy surgery in general (including hemispherectomy) to ensure that such procedures would achieve two goals: 1) eliminating or greatly reducing seizures and 2) causing no or minimal deficits that could impair the patient’s functionality. History and development A hemispherectomy was first performed by the pioneering neurosurgeon Walter Dandy while at The Johns Hopkins Hospital in 1923 for glioblastoma multiforme. The first account of hemispherectomy used to treat epilepsy was in 1938 by McKenzie in Toronto, Canada. The procedure gained further popularity in the 1950s following a successful series of hemispherectomies in 12 children by South African neurosurgeon Rowland Krynauw and its adoption by the world-renowned Canadian neurosurgeons Wilder Penfield and Theodor Rasmussen at the Montreal Neurological Institute. Up until that point, hemispherectomy was done by completely removing one hemisphere, which is now known as anatomical hemispherectomy. In the following decades, complications in patients who underwent anatomical hemispherectomy began to be recognized including cerebral hemosiderosis (accumulation of blood in cerebral cortex) and hydrocephalus (excessive cerebrospinal fluid accumulation), which led to a decline in its usage. These complications were believed to be related to the significant amount of brain removed during surgery; therefore, to reduce the risk of complication, Rasmussen developed a modified technique termed “subtotal hemispherectomy” in 1968 that include removing two-thirds to three-quarters