Music performed a cappella (/ˌɑː kəˈpɛlə/ AH kə-PEL-ə, UK also /ˌæ kəˈpɛlə/ AK ə-PEL-ə, Italian: [a kkapˈpɛlla]; lit. 'in [the style of] the chapel'), less commonly spelled acapella in English, is music performed by a singer or a singing group without instrumental accompaniment. The term a cappella was originally intended to differentiate between Renaissance polyphonic and Baroque concertato musical styles. In the 19th century, a renewed interest in Renaissance polyphony, coupled with an ignorance of the fact that vocal parts were often doubled by instrumentalists, led to the term coming to mean unaccompanied vocal music. The term is also used, rarely, as a synonym for alla breve.

Research suggests that singing and vocables may have been what early humans used to communicate before the invention of language. The earliest piece of sheet music is thought to have originated from times as early as 2000 BC, while the earliest that has survived in its entirety is from the first century AD: a piece from Greece called the Seikilos epitaph.

A cappella music was originally used in religious music, especially church music as well as anasheed and zemirot. Gregorian chant is an example of a cappella singing, as is the majority of secular vocal music from the Renaissance. The madrigal, up until its development in the early Baroque into an instrumentally accompanied form, is also usually in a cappella form. The Psalms note that some early songs were accompanied by string instruments, though Jewish and Early Christian music was largely a cappella; the use of instruments has subsequently increased within both of these religions as well as in Islam.

The polyphony of Christian (predominantly Catholic) a cappella music began to develop in Europe around the 9th century AD with the practice of organum, reaching its height between the 14th and 16th centuries with compositions by composers of the Franco-Flemish school (such as Guillaume Du Fay, Johannes Ockeghem, and Josquin des Prez). The early a cappella polyphonies were sometimes doubled with other instruments, which were often wind or string instruments, or organs. By the 16th century, a cappella polyphony had further developed, but gradually, the cantata began to take the place of a cappella forms. Sixteenth-century a cappella polyphony, nonetheless, continued to influence church composers throughout this period and to the present day. Recent evidence has shown that some of the early pieces by Palestrina, such as those written for the Sistine Chapel, were intended to be accompanied by an organ "doubling" for some or all of the voices.

Other composers that utilized the a cappella style, if only for the occasional piece, were Claudio Monteverdi and his Lagrime d'amante al sepolcro dell'amata (A lover's tears at his beloved's grave), which was composed in 1610, and Andrea Gabrieli when upon his death many choral pieces were discovered, one of which was in the unaccompanied style. Learning from the preceding two composers, Heinrich Schütz utilized the a cappella style in numerous pieces; chief among these were the pieces in the oratorio style, which were traditionally performed during the Easter week and dealt with the religious subject matter of that week, such as the Passion. Five of Schutz's Historien were Easter pieces, and of these the latter three, which dealt with the passion from three different viewpoints, those of Matthew, Luke and John, were all done a cappella style. The parts of the crowd were sung while the solo parts which were the quoted parts from either Christ or the authors were performed in a plainchant.

In the Byzantine Rite of the Eastern Orthodox Church, Eastern Lutheran Churches and the Eastern Catholic Churches, the music performed in the liturgies is exclusively sung without instrumental accompaniment. Early Russian musika which started appearing in the late 17th century, in what was known as khorovïye kontsertï (choral concertos) made a cappella adaptations of Venetian-styled pieces, such as the treatise, Grammatika musikiyskaya (1675), by Nikolai Diletsky. Divine Liturgies and Western Rite Masses composed by famous composers such as Peter Tchaikovsky, Sergei Rachmaninoff, Alexander Arkhangelsky, and Mykola Leontovych are examples.

Instruments have divided Christendom since their introduction into worship. They were considered a Roman Catholic innovation, not widely practiced until the 18th century, and were opposed vigorously in worship by a number of Protestant Reformers, including Martin Luther, Ulrich Zwingli, John Calvin, and John Wesley. Opponents of musical instruments in the Christian worship believe that such opposition is supported by the Christian scriptures and Church history. There is no reference to instrumental music in early church worship in the New Testament, or in the worship of churches for the first six centuries. Several reasons have been posited throughout church history for the absence of instrumental music in church worship.

Those who do not adhere to the regulative principle of interpreting Christian scripture, believe that limiting praise to the unaccompanied chant of the early church is not commanded in scripture, and that churches in any age are free to offer their songs with or without musical instruments. Those who subscribe to this interpretation believe that since the Christian scriptures never counter instrumental language with any negative judgment on instruments, opposition to instruments instead comes from an interpretation of history. There is no written opposition to musical instruments in any setting in the first century and a half of Christian churches (33–180 AD). The use of instruments for Christian worship during this period is also undocumented. Toward the end of the 2nd century, Christians began condemning the instruments themselves. Those who oppose instruments today believe these Church Fathers had a better understanding of God's desire for the church, but there are significant differences between the teachings of these Church Fathers and Christian opposition to instruments today.