Being possessed by demons or evil spirits is one of the oldest ways of explaining bodily and mental disorders. The article briefly mentions some contributions from other disciplines, but considers in particular psychiatry's and medicine's approach throughout history. During the middle ages of Europe possession (and witchcraft) was considered just one out of several causes of mental illness. Astrological theories prevailed, in addition to the humoral theories of medicine. In addition distinctions were made between eccentricity, madness and religious visions and revelations. A large number of the alleged witches and possessed persons who were burned probably had visible mental disturbances. Today's psychiatry does not refer symptoms of possession to any specific category, but usually classifies this as some kind of psychotic disturbance of thought. Exorcism of evil spirits by Jesus Christ is described often in the Gospels. Possession was the only "available" concept for a bundle of neuro-psychiatric disorders: dissociative states, psychoses and epilepsy. To day, the terminology and "diagnostic" principles have been taken over by fundamentalistic groups, who practise exorcism on persons with (probably) minor mental problems and symptoms. The author criticises this activity.