Holy Communion is one of the seven sacraments in the Catholic Church. With Baptism and Confirmation, they constitute the sacraments of Initiation. Similarly, with the Word of God, they constitute the two indispensable pillars upon which the Church is built. It is the “fount and apex of the whole Christian life” (LG 11). It is named Holy Eucharist because it is an action of thanksgiving to God. It recalls God’s work of creation, redemption, and sanctification. The Eucharistic elements, bread and wine become, by the prayer of consecration and the invocation of the Holy Spirit, Christ's Body and Blood through an act appropriately known as transubstantiation. The term emphasizes the conversion of the total substance of bread and wine into the entire substance of the Body and Blood of Christ. When the bread and wine are consecrated at Mass, they are no longer bread and wine; they have become instead the Most Precious Body and Blood of Christ by the power of the Holy Spirit in accordance with the words of Christ. The empirical appearances and attributes remain the same, but the underlying reality changes. Therefore, the doctrine of transubstantiation teaches without ambiguity that in the Holy Communion, the Body and Blood, together with the soul and divinity, of the Lord Jesus Christ is truly, really, and substantially contained. How is this understood and what is its implication theologically? In an attempt to elucidate this problem, this work seeks first to highlight the theology of the Holy Eucharist within the context of the ecclesiology of Communion, and second, through some theological themes: sacred memorial and sacrificial banquet; eschatological meal. The third and final part treats the theme of real presence under the rubrics of Transubstantiation. Keywords: Transubstantiation, Eschatological Meal, Memorial, Real Presence, Communion, Eucharistic conversion.