In Christian theology, the term Body of Christ (Latin: Corpus Christi) has two main but separate meanings: it may refer to Jesus Christ's words over the bread at the celebration of the Jewish feast of Passover that "This is my body" in Luke 22:19–20 (see Last Supper), or it may refer to all individuals who are "in Christ" (1 Corinthians 12:12–14; see Christian Church).

As used by Paul in the Pauline epistles "Body of Christ" refers to all individuals who "heard the word of truth, the gospel of your salvation, believed in him, were sealed with the promised Holy Spirit" (Ephesians 1:13), "are being built together into a dwelling place for God by the Spirit" (Ephesians 2:22), are "joined and held together by every joint with which it is equipped, when each part is working properly, makes the body grow so that it builds itself up in love" (Ephesians 4:16).

There are significant differences in how Christians understand the term as used by Christ at the Last Supper and as developed in Christian theology of the Eucharist. For some it may be symbolic, for others it becomes a more literal or mystical understanding.

In Catholic theology the use of the phrase "mystical body" distinguishes the mystical body of Christ, i.e. the Church, from the physical body of Christ, and from a "moral body", such as any club with a common purpose. In Eastern Orthodoxy, the term "mystical body of Christ" is also applied to the Eastern Orthodox Church in the sense that "mystical union with Christ is a reality in his Church".

A belief in the real presence of Christ in the Eucharist is taught in Catholicism, Eastern Orthodoxy, Oriental Orthodoxy, the Church of the East, the Moravian Church, Lutheranism, Anglicanism, Methodism and Reformed Christianity, though each tradition teaches a unique view of the doctrine. Efforts at mutual understanding of the range of beliefs by these Churches led in the 1980s to consultations on Baptism, Eucharist and Ministry by the World Council of Churches.

While teaching that in the bread consecrated in the Eucharist there is absolutely no change open to the senses or to scientific investigation, the Catholic Church supports the real presence, i.e. that the reality of the bread is changed into that of the body of Christ. The Church teachings refer to this change as one of the "substance" or "transubstantiation". It rejects the Lollard doctrine of "consubstantiation", which suggests that the substance or reality of the bread remains after the consecration, instead of being converted or changed into that of the body of Christ. At the same time, the Church holds that all that can be examined either directly or by scientific investigation – what in Aristotelean philosophy are called the "accidents" (as opposed to the reality) – remains quite unchanged.

In the Roman Rite, the priest or other minister who gives the consecrated host to a communicant says: "The body of Christ".

Since the consecrated bread is believed to be the body of Christ and sacred, what remains of the host after celebration of Mass is kept in the church tabernacle. This is primarily for the purpose of taking Communion to the sick, but also to serve as a focal point for private devotion and prayer. On appropriate occasions, there may be public Eucharistic adoration.