This article addresses the validity of viewing demonic possession as a phenomenon distinct from any other form of pathology. It does so by briefly examining the historical relationship of demon possession and other forms of illness; reviewing some of the psychological research into the phenomena of possession; and then presenting a description of possession derived from a study of fourteen possessed individuals. The article concludes that possessions do exist as a phenomena independent of the current commonly accepted forms of psychopathology. A diagnostic description of possession is then presented to enable a greater ability to differentiate cases of possession from the present categories of the DSM-III.