An auxiliary bishop is a bishop assigned to assist the diocesan bishop in meeting the pastoral and administrative needs of the diocese. Auxiliary bishops can also be titular bishops of sees that no longer exist.
In Catholic Church, auxiliary bishops exist in both the Latin Church and in the Eastern Catholic Churches. The particular duties of an auxiliary bishop are given by the diocesan bishop and can vary widely depending on the auxiliary bishop, the ordinary, and the needs of the diocese. In a larger archdiocese, they might be in assigned to serve a portion of the archdiocese (sometimes called deaneries, regions, or vicariates) or to serve a particular population such as immigrants or those of a particular heritage or language. Canon law requires that the diocesan bishop appoint each auxiliary bishop as vicar general or episcopal vicar of the diocese.
In Eastern Orthodox Churches, auxiliary bishops are also called vicarian bishops or simply vicar-bishops. In Serbian Orthodox church, the office of auxiliary (vicar) bishop is entrusted to titular bishops, who are assigned with assisting diocesan bishops in various aspects of diocesan administration. For example, Teodosije Šibalić (titular Bishop of Lipljan) was appointed auxiliary bishop to the Eparchy of Raška and Prizren in 2004.