In the Catholic liturgy the expression ad orientem (Latin for "to the east") is the eastward orientation of a priest celebrating Mass. This orientation has been described as the "cosmic sign of the rising sun which symbolizes the universality of God." 
The expression ad orientem is often used instead to mean facing the apse or wall behind the altar, with priest and people looking in the same direction, even if not to the east, even when they have their backs to the east. Thus, when Pope Benedict XVI celebrated Mass in the Sistine Chapel on 13 January 2008, his choice of orientation was described as ad orientem, although he was in fact facing the west wall of the chapel.
The ad orientem orientation is opposed to that called versus populum, in which the celebrating priest faces the people.
In 7th century England, Catholic churches were built so that on the very feast day of the saint in whose honor they were named, Mass could be offered on an altar while directly facing the rising sun.
It was in the 8th or 9th century that the position whereby the priest faced the apse, not the people, when celebrating Mass was adopted in the Roman Basilicas. This usage was introduced from the Frankish Empire and later became almost universal in the West. However, the Tridentine Roman Missal continued to recognize the possibility of celebrating Mass "versus populum" (facing the people), and in several churches in Rome, it was physically impossible, even before the twentieth-century liturgical reforms, for the priest to celebrate Mass facing away from the people, because of the presence, immediately in front of the altar, of the "confession" (Latin: confessio), an area sunk below floor level to enable people to come close to the tomb of the saint buried beneath the altar.
The present-day Roman Missal does not forbid the ad orientem position for the priest when saying Mass and only requires that in new or renovated churches the facing-the-people orientation be made possible: "The altar should be built apart from the wall, in such a way that it is possible to walk around it easily and that Mass can be celebrated at it facing the people, which is desirable wherever possible." As in some ancient churches the ad orientem position was physically impossible, so today there are churches and chapels in which it is physically impossible for the priest to face the people throughout the Mass.
A letter of 25 September 2000 from the Congregation for Divine Worship and the Discipline of the Sacraments treats the phrase "which is desirable wherever possible" as referring to the requirement that altars be built apart from the wall, not to the celebration of Mass facing the people, while "it reaffirms that the position toward the assembly seems more convenient inasmuch as it makes communication easier ... without excluding, however, the other possibility."
On 13 January 2008, Pope Benedict XVI publicly celebrated Mass in the Sistine Chapel at its altar, which is attached to the west wall. He has since celebrated Mass at the same altar in the Sistine Chapel annually for the Feast of the Baptism of the Lord. On 1 December 2009, he celebrated Mass in the Pauline Chapel in the Apostolic Palace. It was the first time that this Pope publicly celebrated Mass ad orientem on a freestanding altar. He celebrated Mass facing the altar again in the Pauline Chapel on 15 April 2010.
- The Spirit of the Liturgy, Cardinal Joseph Ratzinger, Ad Solem, 2006 p. 64
- Phil Lawler, Ad Orientem, The Single Most Important Reform
- Catholic News Service: Not exactly 'ad orientem'
- The Oxford Dictionary of the Christian Church (Oxford University Press 2005 ISBN 978-0-19-280290-3), article "orientation"
- "When Christians in fourth-century Rome could first freely begin to build churches, they customarily located the sanctuary towards the west end of the building in imitation of the sanctuary of the Jerusalem Temple. Although in the days of the Jerusalem Temple the high priest indeed faced east when sacrificing on Yom Kippur, the sanctuary within which he stood was located at the west end of the Temple. The Christian replication of the layout and the orientation of the Jerusalem Temple helped to dramatize the eschatological meaning attached to the sacrificial death of Jesus the High Priest in the Epistle to the Hebrews" (The Biblical Roots of Church Orientation by Helen Dietz).
- Andrew Louth, "The Body in Western Catholic Christianity," in Religion and the Body, ed. by Sarah Coakley, (Cambridge, 2007) p. 120.
- The Oxford Dictionary of the Christian Church (Oxford University Press 2005 ISBN 978-0-19-280290-3), article "westward position"
- The Oxford Dictionary of the Christian Church (Oxford University Press 2005 ISBN 978-0-19-280290-3), article "eastward position"
- Ritus servandus in celebratione Missae, V, 3
- General Instruction of the Roman Missal, 299
- English translation of Letter of protocol number 2036/00/L and date 25 September 2000
- Isabelle de Gaulmyn, Benedict XVI celebrated a Mass "back to the people" in La Croix, 15 January 2008
- Gregor Kollmorgen, Pope Celebrates Ad Orientem in the Pauline Chapel in New Liturgical Movement, 15 January 2008
- Gregor Kollmorgen, Holy Father Celebrates Mass with the Pontifical Biblical Commission in New Liturgical Movement, 15 April 2010