Exorcism
In Christianity, exorcism is the practice of casting out demons. In Christian practice the person performing the exorcism, known as an exorcist, is often a member of the Christian Church, or an individual thought to be graced with special powers or skills. The exorcist may use prayers and religious material, such as set formulas, gestures, symbols, icons, amulets, etc. The exorcist often invokes God, Jesus and/or several different angels and archangels to intervene with the exorcism. Protestant Christian exorcists most commonly believe the authority given to them by the Father, Son, and Holy Spirit (the Trinity) is the source of their ability to cast out demons.
International Association of Exorcists
Although the membership is restricted and exclusive, by 2000 there were over two hundred members, who meet bi-annually in Rome. The association sends out a quarterly newsletter where members can tell of particularly interesting or difficult cases. An exorcist priest must have the permission of his bishop to join. 
Within the Roman Catholic Church a priest may only do an exorcism with the express consent of his bishop or local ordinary, and only, to the extent necessary, after an examination of the patient by doctors and psychiatrists in order to determine that the affliction has no natural origin. A priest is required by canon law to be devout, knowledgeable, prudent, and possess integrity of life.