The Mass is the central liturgical service of the Eucharist in the Catholic Church, in which bread and wine are consecrated and become the body and blood of Christ. As defined by the Church at the Council of Trent, in the Mass "the same Christ who offered himself once in a bloody manner on the altar of the cross, is present and offered in an unbloody manner". The Church describes the Mass as the "source and summit of the Christian life", and teaches that the Mass is a sacrifice, in which the sacramental bread and wine, through consecration by an ordained priest, become the sacrificial body, blood, soul, and divinity of Christ as the sacrifice on Calvary made truly present once again on the altar. The Catholic Church permits only baptised members in the state of grace to receive Christ in the Eucharist.

Catholic liturgy means the whole complex of official liturgical worship, including all the rites, ceremonies, prayers, and sacraments of the Church, as opposed to private devotions. In this sense the arrangement of all these services in certain set forms is meant. Liturgy encompasses the entire service: prayer, reading and proclamation, singing, gestures, movement and vestments, liturgical colours, symbols and symbolic actions, the administration of sacraments and sacramentals.