A sequence (Latin: sequentia, plural: sequentiae) is a chant or hymn sung or recited during the liturgical celebration of the Eucharist for many Christian denominations, before the proclamation of the Gospel. By the time of the Council of Trent (1543–1563) there were sequences for many feasts in the Church's year.

The sequence had always been sung directly before the Gospel, after the Alleluia. The 2002 edition of the General Instruction of the Roman Missal, however, reversed the order and places the sequence before the Alleluia.

The form of this chant inspired a genre of Latin poetry written in a non-classical metre, often on a sacred Christian subject, which is also called a sequence.

The Latin sequence has its beginnings, as an artistic form, in early Christian hymns such as the Vexilla Regis of Venantius Fortunatus. Venantius modified the classical metres based on syllable quantity to an accentual metre more easily suitable to be chanted to music in Christian worship. In the ninth century, Hrabanus Maurus also moved away from classical metres to produce Christian hymns such as Veni Creator Spiritus.

The name sequentia, on the other hand, came to be bestowed upon these hymns as a result of the works of Notker Balbulus, who popularized the genre in the ninth century by publishing a collection of sequentiae in his Liber Hymnorum. Since early sequences were written in rhythmical prose, they were also called proses (Latin: prosae).

Notker's texts were meant to be sung. In the Latin Mass of the Middle Ages, it became customary to prolong the last syllable of the Alleluia, while the deacon was ascending from the altar to the ambo, to sing or chant the Gospel. This prolonged melisma was called the jubilus, jubilatio, or laudes, because of its jubilant tone. It was also called sequentia, "sequence," because it followed (Latin: sequi) the Alleluia. Notker set words to this melisma in rhythmic prose for chanting as a trope. The name sequence thus came to be applied to these texts; and by extension, to hymns containing rhyme and accentual metre. A collection of sequences was called the Sequentiale.

One well-known sequence, falsely attributed to Notker during the Middle Ages, is the prose text Media vita in morte sumus ("In the midst of life we are in death"), which was translated by Cranmer and became a part of the burial service in the funeral rites of the Anglican Book of Common Prayer. Other well-known sequences include the ninth-century Swan Sequence, Tommaso da Celano's Dies Irae, St. Thomas Aquinas' Pange lingua in praise of the Eucharist, the anonymous medieval hymn Ave maris stella ("Hail, star of the sea!"), and the Marian sequence Stabat Mater by Jacopone da Todi. During the Middle Ages, secular or semi-secular sequences, such as Peter of Blois' Olim sudor Herculis ("The labours of Hercules") were written; the Goliards, a group of Latin poets who wrote mostly satirical verse, used the form extensively. The Carmina Burana is a collection of these sequences.

In the Missal of Pius V (1570) the number of sequences for the entire Roman Rite was reduced to four: