Demonic possession' is held by many belief systems to be the Spirit possession|control of an individual by a demon|malevolent preternatural being. Descriptions of demonic possessions often include erased memories or personalities, convulsions, “fits” and fainting as if one were dying. Other descriptions include access to hidden knowledge (gnosis) and foreign languages (glossolalia), drastic changes in vocal intonation and facial structure, the sudden appearance of injuries (scratches, bite marks) or lesions, and superhuman strength. Unlike in mediumship|channeling, the subject has no control over the possessing entity and so it will persist until forced to leave the victim, usually through a form of exorcism.

Many cultures and religions contain some concept of demonic possession, but the details vary considerably. The oldest references to demonic possession are from the Sumerians, who believed that all diseases of the body and mind were caused by "sickness demons" called ''gidim'' or ''gid-dim''.The priests who practiced exorcisms in these nations were called ''ashipu'' (sorcerer) as opposed to an ''asu'' (physician) who applied bandages and salves. Clay tablet|tablets contain prayers to certain gods asking for protection from demons, while others ask the gods to expel the demons that have invaded their bodies.

Shamanic cultures also believe in demon possession and [shamans perform exorcisms. In these cultures, diseases are often attributed to the presence of a vengeful spiritual being|spirit (or loosely termed demon) in the body of the patient. These spirits are more often the spectres of animals or people wronged by the bearer, the exorcism rites usually consisting of respectful offerings or sacrificial offerings.

Christianity holds that possession derives from the Devil, i.e. Satan or one of his lessor demons. In many Christian belief systems, Satan and his demons are actually fallen angels