More Recent Documents on the Eucharistic Mystery
1. The eucharistic mystery is truly the center of the liturgy and indeed of the whole Christian life. Consequently the Church, guided by the Holy Spirit, continually seeks to understand this mystery more fully and more and more to derive its life from it.
In our day Vatican Council II has stressed several important aspects of this mystery.
Through the Constitution on the Liturgy, after first recalling certain realities about the nature and importance of this sacrament, the Council established the norms for the reform of the rites of the sacrifice of the Mass so that the celebration of this mystery would further the active and full participation of the faithful. In addition, the Constitution broadened the practice of concelebration and communion under both kinds.
In the Constitution on the Church the Council set forth the close connection between the eucharist and the mystery of the Church. In other documents the Council frequently stressed the important place of the eucharistic mystery in the life of the faithful  and its power to shed light on the meaning of human labor and indeed of all creation insofar as in it "natural elements, after being fashioned by human hands, are changed into the glorious body and blood of Christ." 
For many of these pronouncements of the Council Pius XII had prepared the way, especially by his encyclical Mediator Dei.  In his encyclical Mysterium fidei  Pope Paul VI has recalled the importance of certain aspects of eucharistic teaching, especially on the real presence of Christ and the worship due to this sacrament outside Mass.
Need to Attend Simultaneously to the Complete Teaching of these Documents
2. In recent times certain aspects of the traditional teaching on this mystery have been considered more thoroughly and have been presented with new zeal to the devotion of the faithful. Research and practical measures of various kinds, especially in the field of liturgy and Scripture, have provided assistance.
There is, consequently, a need to draw out practical norms from the total teaching of such documents, in order to indicate what the relationship of the Christian people toward this mystery should be so that they may achieve that understanding and holiness which the Council set before the Church as an ideal.
It is important that the eucharistic mystery, fully considered under the many facets of its own reality, appear with the clarity it should have before the minds of the faithful; and also that the relationships that are recognized in church teaching as existing objectively between the various facets of the mystery become reflected in the life and mind of the faithful.
The Most Notworthy Doctrinal Themes in the Recent Documents
3. Among the doctrinal principles formulated in the Church's recent documents concerning the eucharist, it is useful to cite those that follow: they address the attitude of Christians toward this mystery and, therefore, have direct bearing on the purpose of this Instruction.
a. "In the human nature united to himself the Son of God, by overcoming death through his own death and resurrection, redeemed us and refashioned us into a new creation (see Gal 6:15; 2 Cor 5:17). By communicating his Spirit, Christ made us his brothers and sisters, called together from all nations, to be mystically his own Body. In that Body the life of Christ is bestowed on believers, who suffered and was glorified." 
Therefore, "at the Last Supper, on the night he was betrayed, our Saviour instituted the eucharistic sacrifice of his body and blood. He did this in order to perpetuate the sacrifice of the cross throughout the centuries until he should come again and so to entrust to his beloved Bride, the Church, a memorial of his death and resurrection: a sacrament of love, a sign of unity, a bond of charity, a paschal banquet in which Christ is eaten, the heart filled with grace, and a pledge of future glory given to us." 
Hence the Mass, the Lord's Supper, is at once and inseparably:
-the sacrifice in which the sacrifice of the cross is perpetuated;
-the memorial of the death and resurrection of the Lord who said: "Do this in memory of me" (Lk 22:19);
-the sacred banquet in which, through the communion of the body and blood of the Lord, the people of God share the benefits of the paschal sacrifice, renew the New Covenant with us made once and for all by God in Christ's blood, and in faith and hope foreshadow and anticipate the eschatological banquet in the Father's kingdom as they proclaim the death of the Lord "until he comes." 
b. In the Mass, therefore, the sacrifice and sacred meal form part of the same mystery in such a way that the closest bond conjoins the one with the other.
In the sacrifice of the Mass the Lord is offered when "he begins to be sacramentally present as the spiritual food of the faithful under the appearance of bread and wine."  The reason that Christ entrusted this sacrifice to the Church was that the faithful might share in it both spiritually, by faith and charity, and sacramentally, through the sacred meal of communion. A sharing in the Lord's Supper is always a communion with Christ offering himself to the Father for us as a sacrifice. 
c. The celebration of the eucharist at Mass is the action not only of Christ but also of the Church. It is Christ's act because, perpetuating in an unbloody way the sacrifice consummated on the cross,  he offers himself to the Father for the salvation of the world through the ministry of priests.  It is the Church's act because, as the Bride and minister of Christ exercising together with him the role of priest and victim, the Church offers him to the Father and at the same time completely offers itself together with him. 
In this way, especially in the great eucharistic prayer, the Church gives thanks together with Christ to the Father in the Holy Spirit for all the benefits he gives us in creation and in a singular way in the paschal mystery and asks the Father for the coming of his kingdom.
d. Hence no Mass, in fact no liturgical service, is a merely private act, but the celebration of the Church as a society composed of different orders and ministries in which all the members have an active part in keeping with their proper order and office. 
e. The celebration of the eucharist in the sacrifice of the Mass is truly the origin and the purpose of the worship that is shown to the eucharist outside Mass. For the sacred elements that remain after Mass come from the Mass and they are reserved after Mass so that the faithful who cannot be present at Mass may be united to Christ and the celebration of his sacrifice through sacramental communion received with the right dispositions. 
Hence the eucharistic sacrifice is the source and the summit of all the Church's worship and of the entire Christian life.  The faithful participate more fully in this sacrifice of thanksgiving, expiation, petition, and praise not only when they wholeheartedly offer the sacred victim and in him offer themselves to the Father with the priest, but also when they receive the same victim in the sacrament.
f. It should be absolutely clear "that all the faithful show this holy sacrament the worship of adoration that is due to God himself, as has always been the practice recognized in the Catholic Church. Nor is the sacrament to be less the object of adoration on the grounds that it was instituted by Christ the Lord to be received as food."  For even in the reserved sacrament he is to be adored,  because he is substantially present there through the conversion of the bread and wine that, following the Council of Trent,  is most accurately termed transubstantiation.
g. Therefore, the eucharistic mystery must be considered in its entirety, both in the celebration of Mass and in the worship of the sacred elements reserved after Mass in order to extend the grace of the sacrifice. 
The principles stated must be the source of the norms on the practical arrangements of the worship of this sacrament even after Mass and of its correlation with the proper arrangement of the Mass in conformity with the directives of Vatican Council II and of other pertinent documents of the Apostolic See. 
General Intent of This Present Instruction
4. For this reason Pope Paul VI ordered the Consilium for the Implementation of the Constitution on the Liturgy to prepare a special instruction that would issue such practical norms, fitted to contemporary circumstances.
The purpose intended for these norms is both to provide the broad principles for catechesis of the faithful about the eucharistic mystery and to make more understandable the signs through which the eucharist is celebrated as the memorial of the Lord and worshiped in the Church as a lasting sacrament.
For although this mystery has a supreme and unique excellence, namely, the presence of the very author of holiness, nevertheless in common with the other sacraments, it too is the symbol of a sacred reality and the visible expression of an invisible grace.  Hence the more pertinent and clear the signs involved in its celebration and worship, the more surely and effectively will it penetrate the minds and lives of the faithful.