The Solemnity of the Most Holy Body and Blood of Christ (formerly called the Feast of Corpus Christi) is a special celebration in the Roman Catholic Church to commemorate the mystery of the Mass where the outward elements of bread and wine undergo a change called transubstantiation. It is believed that these elements are transformed into the Real Presence of the body and blood of Christ.

In the liturgical year this observance follows the seven Sundays of the Easter season and then the Solemnity of the Most Holy Trinity. It is thought that Corpus Christi is a logical conclusion of what has happened in the preceeding eight weeks.

If faith is required to believe in the resurrection of Christ at Easter, perhaps more is required to affirm that God is a unity in the three Persons of the Trinity. Yet even more is needed to believe that at every mass transubstantiation takes place. It is estimated that only about 30% of practicing Catholics have that much faith.

While transubstantiation has almost from the beginning been an article of Catholic doctrine, there was no special observance to commemorate it in the mass until St. Juliana of Liege (1193-1258), a Belgium nun, pressed for its observance in the universal Church. First the bishop of Liege and finally Pope Urban IV and his successor, Clement V, took up the cause and finally established it as part of the liturgical year.

Two special ceremonies mark the solemnity.