In Christianity, concelebration (from Lat., con + celebrare,to celebrate together) is the presiding of a number of presbyters (priests or ministers) at the celebration of the Eucharist with either a presbyter or bishop as the principal celebrant and the other presbyters and bishops present in the chancel assisting in the consecration of the Eucharist. The concelebrants assist the principal celebrant by reciting the Words of Consecration together with him, thus effecting the change of the eucharistic elements. They may also recite portions of the Eucharistic Prayer. Concelebration is often practiced by ministers/priest of Churches that are in full communion with one another, e.g. the Anglican Communion and the Old Catholic Church.
Concelebration is and has always been common in Eastern Christianity, but in the Roman Catholic Church the practice fell into disuse for several centuries but has been revived with the liturgical reforms of the Second Vatican Council as dictated in the document Sacrosanctum Concilium. Theologians Orlando O. Espín and James B. Nickoloff write that "Eucharastic concelebration has ancient roots (Hippolytus, Apostolic Tradition, early third century) and was practiced primarily as a sign of ecclesial unity of the local church and of union with other churches when eucharistic hospitality was offered to visiting bishops or presbyters."
It is known from early Christian art uncovered through archaeology that concelebration took place in the Early Church in the West, although it is not known precisely when it fell into disuse. It is known that the practice gradually came to be reserved for the greater festival days and other solemn occasions. Columba of Iona, in the 6th century, is recorded to have been requested by a visiting Irish bishop in disguise to celebrate mass with him. The 619 council of Seville ordered that priests could not concelebrate mass with a bishop present. According to Pope Innocent III (r.1198-1216) the cardinals in Rome still concelebrated with the pope on certain feast days. It is known that from the medieval period (particularly following the Black Death when masses for the dead celebrated daily by all priests became the norm, with the exception of the Sundays). By the Second Vatican Council there were only two occasions when concelebration took place in the Roman Rite: