The practice of reserving some of the Blessed Sacrament consecrated at a previous Mass dates back to the earliest days of the Church. Reservation served one purpose only: so that Holy Communion could be taken to the sick and homebound who could not attend the Sunday liturgy. Although the use of the tabernacle to house the Blessed Sacrament was a later development, a sense of the sacredness of the reserved Eucharist always existed and due reverence was always shown to it. This reverence is fitting and follows logically from the fact that once the bread and wine of the Eucharist has been transformed into the Body and Blood of Christ, they remain such. Once the liturgy ends, the Body and Blood remains and can be devoutly consumed at any time; and wherever the Blessed Sacrament is reserved, it is worthy of the worship given to it.
Because the Body and Blood of Christ remain under the signs of bread and wine even after the celebration of a given Mass, it has been always necessary for the Eucharistic elements to be properly reserved and shown due reverence. Over the centuries the practice of adoring or worshipping the reserved Eucharist developed. By the 12th century the ordinary place of reservation of the Blessed Sacrament become the tabernacle, a “box” or other suitable vessel, located in a prominent place in the church: often an attached chapel or a side altar. The placing of the tabernacle on the main altar did not become common until the 15th century; following the Second Vatican Council the older practice of locating the tabernacle elsewhere in the church returned.
In any case, adoration of the reserved Blessed Sacrament developed over the centuries. The term adoration refers to the worship given the Blessed Sacrament reposed within tabernacle. Adoration can take place any time access to the place of reservation is available, and can be done individually or in a group setting. It can be for personal edification or the building up of a group or association of the faithful. No particular rite or regulations exist for adoration; it can take place as a simple period of private prayer and meditation, or it can include specific prayers and devotions such as the Rosary, various litanies, and so on.
Adoration can also take place within the rite of Exposition, which involves the removal of the Blessed Sacrament from the tabernacle and its display in either a monstrance (from Latin: “to show”), a pyx, or ciborium. Exposition has a highly public and communal aspect, and involves a carefully regulated liturgy found in the United States in the Order for the Solemn Exposition of the Holy Eucharist (OSEHE) and in the Roman Ritual, Holy Communion and Worship of the Eucharist Outside Mass, Chapter III.