Sudden unexpected death in epilepsy (SUDEP) is the sudden unexpected death of a seemingly healthy individual with epilepsy. SUDEP is the commonest cause of death directly attributable to epilepsy, and most often occurs at or around the time of a seizure and during sleep. Sleep, respiration, arousal responses, and caudal brainstem serotoninergic neurons probably play roles in SUDEP, but more research is needed to understand these relationships. This article reviews the medical literature on the epidemiology, risk factors, and preventive measures for SUDEP in people with epilepsy, and discusses the roles of sleep, respiration, impaired autonomic functioning, and nocturnal seizures.