This decree explains the Church’s understanding of the Mass as the sacrificial memorial of Christ’s Passion and Resurrection, in which Christ is truly present and the ordained priest acts in his person. It presents the Eucharist as the center of Christian life, involving the whole Church: bishops, priests, deacons, instituted and lay ministers, and the faithful, whose conscious, active, reverent participation is both a right and a duty. The document also emphasizes continuity with the Roman tradition, while defending reforms that restore ancient sources, support catechesis, permit vernacular languages, and provide pastoral flexibility. The instruction then establishes detailed norms for every form of celebration: the Introductory Rites, Liturgy of the Word, Eucharistic Prayer, Communion, and concluding rites; the roles of ministers; concelebration; celebrations with one minister; Communion under both kinds; and the treatment of sacred vessels, vestments, gestures, music, silence, and liturgical books. It prescribes suitable arrangements for churches, altars, ambos, tabernacles, and sacred art, stressing noble simplicity, visibility, reverence, and unity. Finally, it regulates the selection of readings, prayers, chants, calendars, funerary celebrations, translations, and legitimate regional adaptations, assigning oversight to diocesan bishops and bishops’ conferences under approval of the Apostolic See while preserving the Roman Missal as a sign of unity.