Exorcism
In Catholic Christianity, exorcisms are performed in the name of Jesus Christ. A distinction is made between a formal exorcism, which can only be conducted by a priest during a baptism or with the permission of a bishop, and "prayers of deliverance" which can be said by anyone.
The Catholic rite for a formal exorcism, called a "Major Exorcism", is given in Section 11 of the Rituale Romanum. The Ritual lists guidelines for conducting an exorcism, and for determining when a formal exorcism is required. Priests are instructed to carefully determine that the nature of the affliction is not actually a psychological or physical illness before proceeding.
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.