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.
Exorcist
The office of Exorcist was not a part of the sacrament of Holy Orders but as a sacramental was instead first conferred on those who had the special charism to perform its duties and later to those studying for the priesthood. By the twentieth century, the order had become purely ceremonial. As a minor order, exorcists wore the surplice.