The Introit is part of the opening of the liturgical celebration of the Eucharist for many Christian denominations. In its most complete version, it consists of an antiphon, psalm verse and Gloria Patri, which are spoken or sung at the beginning of the celebration. It is part of the proper of the liturgy: that is, the part that changes over the liturgical year.

An antiphon is a short chant in Christian ritual, sung as a refrain. The texts of antiphons are usually taken from the Psalms or Scripture, but may also be freely composed. Their form was favored by St Ambrose and they feature prominently in Ambrosian chant, but they are used widely in Gregorian chant as well. They may be used during Mass, for the Introit, the Offertory or the Communion. They may also be used in the Liturgy of the Hours, typically for Lauds or Vespers.