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.
Gabriele Amorth
Edward Peters, a professor of canon law, finds Amorth's claim to have personally performed 30000 exorcisms over 90 years "astounding". Even accepting Amorth's claim that only 94 of his 30000 exorcisms represented full-blown possession, that would have required roughly one case a month to be thoroughly examined and processed over nine years with hardly a break. Amorth believes that a person may be possessed by more than one demon at once, sometimes numbering in the thousands, which is what accounts for the high number of reputed demons exorcised.