Eucharistic adoration is a practice in the Roman Catholic Church and in some Anglican churches, in which the Blessed Sacrament is exposed to and adored by the faithful. When this exposure and adoration is constant (that is, twenty-four hours a day), it is called perpetual adoration. In a parish, this is usually done by volunteer parishioners; in a monastery or convent, it is done by the resident monks or nuns.
The practice of adoration traces its roots to the fact that in monasteries and convents the Blessed Sacrament was an integral part of the structure of cloistered life. From the beginning of community life the unconsecrated bread and wine were originally kept in a special room, just off the sanctuary but separated from the church where the Divine Liturgy or Mass was offered, stemming from the standard practice of having a skeuophylakion which was found in cathedral and parish churches, an architecturally separate building or room where the deacons prepared the bread and wine before the liturgy in a service developed in the East called the proskomedia.
One of the first possible references to reserving the Blessed Sacrament for adoration is found in a life of St. Basil (who died in 379). Basil is said to have divided the Eucharistic Bread into three parts when he celebrated the Divine Liturgy in the monastery. One part he consumed, the second part he gave to the monks, and the third he placed in a golden dove suspended over the altar http://www.therealpresence.org/eucharst/manual/section1/sec1_12a.pdf. It is more likely, however, that this separate portion was simply for the purpose of reserving the sacrament for distribution in contexts in which a communicant could not attend the Divine Liturgy, which is a standard practice throughout the ancient churches, even those who do not practice extra-liturgical Eucharistic adoration. In Eastern Christianity, the sort of extra-liturgical adoration which developed in the West has never been part of the Eastern liturgy which St. Basil celebrated, but a liturgy for adoration does exist, involving psalms and placing a covered diskos with the Sacred Species on the altar. This is befitting the Eastern custom of veiling those things deemed sacred from human eyes.
The Franciscan archives credit Saint Francis of Assisi (who died in 1226) for starting Eucharistic Adoration in Italy. The lay practice of adoration formally began in Avignon, France on September 11, 1226. To celebrate and give thanks for the victory over the Albigensians in the later battles of the Albigensian Crusade, King Louis VII asked that the sacrament be placed on display at the Chapel of the Holy Cross. The overwhelming number of adorers brought the local bishop, Pierre de Corbie, to suggest that the exposition be continued indefinitely. With the permission of Pope Honorius III, the idea was ratified and the adoration continued there practically uninterrupted until the chaos of the French Revolution halted it from 1792 until the efforts of the Confraternity of Penitents-Gris brought it back in 1829. Twenty years later, the Venerable Leo Dupont initiated the nightly adoration of the Blessed Sacrament in Tours in 1849, from where it spread within France.
The adoration of the Eucharist within France grew in this period and there were interactions between Catholic figures who were enthusiastic about spreading the Eucharist e.g. Leo Dupont and Saint Peter Julian Eymard, who formed the Congregation of the Blessed Sacrament in 1858. The same year, Eyemard (also known as the Apostle of the Eucharist) and sister Marguerite Guillot formed the Servants of the Blessed Sacrament which now maintains houses on several continents where continuous Eucharistic adoration takes place. Interestingly, this time period in France saw the growth of a parallel Catholic devotion, namely the Devotion to the Holy Face of Jesus which was started in 1844 in Tours by Sister Marie of St Peter, was promoted by Leo Dupont and was approved by Pope Leo XIII in 1885.
Mother Mechtilde of the Blessed Sacrament pioneered perpetual adoration of the Eucharist on request of Père Picotte. The Benedictine convent, founded for this purpose, opened in France on March 25, 1654.
Another common early practice of adoration is Quarantore an exercise of devotion in which continuous prayer is made for forty hours before the exposed Blessed Sacrament. This is said to have started in Milan in May 1537.
At years and counting, the Franciscan Sisters of Perpetual Adoration have been praying nonstop longer than anyone in the United States. The practice began on August 1, 1878, at 11 a.m. and continues to this date.
The World's largest Monstrance can be found in Chicago where an adoration chapel is under construction which will be the focus of 24-hour Eucharistic Adoration where there will be no liturgies or vocal prayers, either by individuals or groups as the space will be strictly meant for private meditation and contemplation. This Sanctuary devoted to the The Divine Mercy is being constructed adjacent to Church of St. Stanislaus Kostka, one of the city's famed Polish Cathedrals.
Adoration is a sign of devotion to and worship of Jesus Christ, who is believed by many Christians to be present Body, Blood, Soul, and Divinity, under the appearance of the consecrated host, represented by hosts or bread.
See main article: Eucharist (Catholic Church).
In the Roman Catholic tradition, at the moment of Consecration the elements (or "gifts" as they are termed for liturgical purposes) are transformed (Transubstantiation) into the actual Body and Blood of Christ. Catholic doctrine holds that the elements are not only spiritually transformed, but rather are actually (substantially) transformed into the Body and Blood of Christ. It is held that although the elements retain the appearance or "accidents" of bread and wine, they are indeed the actual Body, Blood, Soul, and Divinity of Christ. This is one form of the doctrine of Real Presence - the actual, substantive presence of Jesus in the Eucharist. At the point of Consecration, the act that takes place is a double miracle: 1) that Christ is present in a physical form and 2) that the bread and wine have truly, substantially become Jesus' Body and Blood. Because Roman Catholics believe that Christ is truly present (Body, Blood, Soul and Divinity) in the Eucharist, the reserved sacrament serves as a focal point of adoration.
Opinions on the nature of the Eucharist and thus on the propriety of adoration of the Blessed Sacrament vary in the Anglican tradition (see Anglican Eucharistic theology), but many Anglo Catholics practice adoration in the manner of the Roman Catholic Church. Others celebrate Benediction of the Blessed Sacrament, which is not unlike Eucharistic adoration.
Lutheran Eucharistic adoration is almost always limited in duration to the communion service because Lutheran tradition does not include reservation of the Sacrament. However, at this time, in North America, the Evangelical Community Church-Lutheran and some other small Churches in the Lutheran Evangelical Catholic Tradition (High Church Lutheran), do reserve the Sacrament, and strongly encourage Eucharistic adoration without requiring it.
Historically in Lutheranism there have been two parties regarding Eucharistic adoration: Gnesio-Lutherans, who followed Martin Luther's view in favor of adoration and Philippists who followed Philipp Melanchthon's view against it. Although Luther did not approve of the Feast of Corpus Christi  , he wrote a treatise "The Adoration of the Sacrament" (Von anbeten des sakraments des heyligen leychnahms Christi, 1523) where he defended adoration but desired that the issue not be forced. After the death of Martin Luther, further controversies developed including Crypto-Calvinism and the second Sacramentarian controversy, started by Gnesio-Lutheran Joachim Westphal. Philippist understanding of the Real Presence without adoration through time became dominant in Lutheranism, although it is not in accordance with Luther's teaching. German theologian Andreas Musculus can be regarded as one of the warmest defenders of Eucharistic adoration in early Lutheranism  .
The host is displayed in a monstrance, typically placed on an altar. The Blessed Sacrament may not actually be exposed, but left in a ciborium, which is likewise placed on an altar. This exposition usually occurs in the context of a service of Benediction or similar service of devotions to the Blessed Sacrament. In services of perpetual adoration, parishioners volunteer to attend for a certain period of time, typically an hour, around the clock. Because of the difficulty of maintaining twenty-four hour attendance, many parishes no longer provide perpetual adoration. In many parishes, the Blessed Sacrament is reserved in an enclosed tabernacle so that the faithful may pray in its presence without the need for volunteers to be in constant attendance (as must be the case when the Blessed Sacrament is exposed).
Many Christians throughout history have criticized Eucharistic adoration, some considering it a form of idolatry. Adoration may be seen as the abrogation of the command to adore God alone, as commanded in Leviticus. They see the adoration of any other objects, including the sacred instruments of His Grace, such as the Body and Blood as idolatry.
Critics draw a distinction between the irreducible risen physical Jesus, and the reducible elements of his body. They point as an example to the third day after the Crucifixion. Although Jesus's blood still drenched the cross and the tomb clothes , the angel states: "He is not here, for He has risen..." Gospel of Matthew 28:6. Similarly the Eucharistic Blood and Body are elements proceeding from Christ, not the irreducible Person Himself. Roman Catholics draw much the same distinction between the irreducible personhood of a man, and the parts of his body. Destruction of a body part (e.g., amputation) is not destruction of the person himself. Similarly, blood used in a blood transfusion derives from its donor, but is not the donor himself. The Catholic Church, however, asserts that the Eucharist contains the fullness of Christ's body, and blood, soul and divinity, not just the proceeding elements from Christ.
Perhaps the most common criticism of the practice of Eucharistic adoration is that it isolates the Eucharist from its desired purpose, namely, communion. The Eucharist is removed from its context as the communion of the Church with Christ and places Him at a distance, objectifying the Eucharist in a manner not consistent with the rites during which it is consecrated.
As reported by Roman Catholic priest, Father Al Kimel in his blog Pontifications, historian and writer Michael McGuckian discusses this problem in his book “The Eucharist in the West.” "During the first millennium of the Church, East and West shared a common understanding of Eucharistic devotion. “During this period,” McGuckian notes, “the Eucharist was understood dynamically and, in common with all the other sacraments, the focus was on its effect in the sanctification of the people.” But in the eleventh century we see a change of attitude within the Western Church. Devotion began to be focused on the Eucharistic gifts as the objective presence of the risen Christ. The Host began to be elevated during the liturgy for the purpose of adoration. In the thirteenth century the Feast of Corpus Christi was instituted. From this point devotion to the Blessed Sacrament, both within and outside the Mass, became central in the piety of Latin Christians. Seeing Christ rather than feeding upon Christ became the norm. Adoration replaced communion.
The adoration of the Blessed Sacrament outside of Mass has been associated with a large amount of Catholic writings and inspirations, e.g. significant portions of the writings of the Venerable Concepcion Cabrera de Armida were reported as having been based on her adorations of the Blessed Sacrament . Cabrera de Armida did not represent her writings as interior locutions or visions of Jesus and Mary but as her meditations and inspirations during Eucharistic adoration.