Versus populum (Latin for "towards the people") is the liturgical stance of a priest who, while celebrating Mass, faces the people from the other side of the altar. The opposite stance, that of a priest facing in the same direction as the people, is today called ad orientem (literally, "towards the east" − even if the priest is really facing in some other direction) or ad apsidem ("towards the apse" − even if the altar is unrelated to the apse of the church or even if the church or chapel has no apse).

In the early history of Christianity it was considered the norm to pray facing the geographical east. From the middle of the 17th century, almost all new Roman Rite altars were built against a wall or backed by a reredos, with a tabernacle placed on the main altar or inserted into the reredos. The altars of Evangelical-Lutheran churches were built against the eastern wall of the church, likewise. This meant that the priest turned to the people, putting his back to the altar, for a few short moments at Mass.

The celebration of the Mass versus populum was suggested in 1526 by Martin Luther in the German Mass who stated that "the priest should always turn himself towards the people as, without doubt, Christ did at the Last Supper" though he noted that immediately implementing this liturgical stance was not compulsory, and that versus populum might become the normative practice with time. After the Second Vatican Council, Catholic churches "pulled the altar from the wall and made it appear to be a table" and Evangelical-Lutheran churches constructed after that time generally followed the same practice, which allowed the presider "to face the congregation as a gathered community".

It has been said that the reason the Pope always faced the people when celebrating Mass in St Peter's was that early Christians faced eastward when praying and, due to the difficult terrain, the basilica was built with its apse to the west. Some have attributed this orientation in other early Roman churches to the influence of Saint Peter's. However, the arrangement whereby the apse with the altar is at the west end of the church and the entrance on the east is found also in Roman churches contemporary with Saint Peter's (such as the original Basilica of Saint Paul Outside the Walls) that were under no such constraints of terrain, and the same arrangement remained the usual one until the sixth century. According to Klaus Gamber, in this early layout the people were situated not in the central nave but in the side aisles of the church and, while the priest faced both the altar and east throughout the Mass, the people faced the altar (from the sides) until the high point of the Mass, when they would turn to face east, the direction in which the priest was already facing. This view is strongly criticized on the grounds of the unlikelihood that, in churches where the altar was to the west, they would turn their backs on the altar (and the priest) at the celebration of the Eucharist.

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 Rome, under the influence of the Frankish Empire, where it had become general. However, 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 best-known such "confession" is that in St Peter's Basilica, but many other churches in Rome have the same architectural feature, including at least one, the present Basilica of Saint Paul Outside the Walls, which, although the original Constantinian basilica was arranged like St Peter's, is oriented since 386 in such a way that the priest faces west when celebrating Mass.

The earliest Christian churches were not built with any particular orientation in mind, but by the fifth century it became the rule in the Eastern Roman Empire to have the altar at the east end of the church, an arrangement that became normal but not universal in northern Europe. The old Roman custom of having the altar at the west end and the entrance at the east was sometimes followed as late as the 11th century even in areas under Frankish rule, as seen in Petershausen (Constance), Bamberg Cathedral, Augsburg Cathedral, Regensburg Cathedral, and Hildesheim Cathedral (all in present-day Germany). In the east also, the original Constantinian Church of the Holy Sepulchre in Jerusalem had its apse to the west until it was Byzantinized in 1048.

The Augustinian Canon Pius Parsch as a military chaplain in World War I and later as priest at St. Gertrude's in Austria popularized what was called a "liturgical Mass" that was said Versus populum. While the practice was permitted, it was considered liturgically advanced at the time.

In the Roman Rite of the Catholic Church, the altar is "the center of thanksgiving that the Eucharist accomplishes" and the point around which the other rites are in some manner arrayed. Its importance was made evident by Romano Guardini (1885–1968), about whom Robert R. Kuehn wrote: "with him [Guardini] on the altar, the sacred table became the center of the universe" [...] The impact of the sacred action was all the more profound because Guardini celebrated the Mass versus populum – facing the people."