Professionally, I believe that most people who claim to be possessed or show behavior others believe to be caused by possession, need to have a thorough psychological workup. Various types of Psychiatric disorders need to be considered especially Psychotic disorders such as schizophrenia and various Dissociative disorders as well. Then there are the ones who claim possession as a way to avoid responsibility for their own actions. (i.e. "I robbed the liquor store because I was possessed") Umm, yeah right
Now personally, I believe in the paranormal and I think that true possession is possible but VERY rare. I've had experiences that have forced me to believe in things I never thought were possible and that others in my profession think is nonsense. I know for a fact that spirits exist and I think that one day science will prove it. And if spirits (or ghosts) are real, then true possession doesn't seem like such an impossibility.
I would love to see someone start a thread on what paranormal experiences they have had.