Orthodox Church, one of the three major branches of Christianity. In historical continuity with the communities created by the apostles of Jesus Christ in the eastern Mediterranean region, the Orthodox church spread by missionary activity throughout Eastern Europe. The Orthodox church has also established communities in Western Europe, the western hemisphere, and, more recently, Africa and Asia. It currently has more than 174 million adherents throughout the world.

The Orthodox church is a fellowship of independent churches, each governed by its own head bishop. These churches share a common faith, common principles of church policy and organization, and a common liturgical tradition. Only the languages used in worship and minor aspects of tradition differ from country to country. The head bishops of the churches are presidents of episcopal synods, which, in each church, constitute the highest authority. Among the various churches, a "primacy of honor" belongs to the patriarch of Constantinople (now Istanbul, Turkey), because the city was the seat of the Byzantine Empire, which between AD 320 and 1453 was the center of Eastern Christendom. His authority has never been comparable to that exercised in the West by the Roman pope.

In its doctrinal statements and liturgical texts, the Orthodox church strongly affirms the authority of the ecumenical councils of the first millennium of Christian history, at which East and West were represented together. These councils defined the basic Christian doctrines on the Trinity, on Christ, and on the Virgin Mary, who is venerated as the Mother of God. The doctrine of seven sacraments is generally accepted in the Orthodox church. The central sacrament is the Eucharist; the others are baptism; confirmation; penance; Holy Orders; marriage; and anointment of the sick. A concern for continuity and tradition is characteristic of Orthodoxy. The Orthodox Church views the apostolic succession of bishops as the guardians and witnesses of a tradition that can be traced back without interruption to the apostles. This succession unites the sacramental community of local churches in the community of faith.

The Orthodox liturgy, created primarily in Byzantium and translated into many languages, preserves texts and forms dating from the earliest Christian church. One of the major characteristics of Orthodox worship is a great wealth of hymns, which mark the various liturgical cycles. Inseparable from the liturgical tradition, religious art is seen by Orthodox Christians as a form of pictorial confession of faith and as a channel of religious experience. This central function of icons— unparalleled in other Christian tradition— received its full definition following the end of the iconoclastic movement in Constantinople in 843. Iconoclasm rejected icons as idols; Orthodox theologians argued that the icons of Christ and the saints provide direct personal contact with the holy persons represented on them. The victory over iconoclasm led to the widespread use of iconography in the Christian East. Monasticism also plays a central role in the Orthodox Church.

Constantinople remained the center of Orthodox Christianity during most of the Middle Ages (5th century to 15th century). Many Slavic nations, including the Bulgarians, the Russians, and the Serbs, were converted to Orthodoxy during this time. Tensions periodically arose between Constantinople and Rome after the 4th century. A number of issues and doctrinal differences became controversial, including the Orthodox practices of allowing married men into the priesthood and of using unleavened bread in the Eucharist. Whereas the East invoked the authority of councils, where the local churches spoke as equals, the papacy in the West considered itself the ultimate judge in matters of faith and discipline. It is often assumed that the final schism occurred in 1054 after the patriarch Michael Cerularius and papal legates issued mutual anathemas of excommunication. The schism, however, actually took the form of a gradual estrangement. Several attempts at reunion ended in failure.

The Orthodox church in Russia declared its independence from Constantinople in 1448. Except for the brief reign of Patriarch Nikon in the mid-1600s, the patriarchs of the Russian church were entirely subordinate to the czars. In 1721 Czar Peter the Great abolished the patriarchate, and thereafter the church was governed through the imperial administration. The patriarchate was reestablished in 1917, at the time of the Russian Revolution, but the church was violently persecuted by the Communist government. As the Soviet regime became less repressive and, in 1991, broke up, the church showed signs of renewed vitality. The Orthodox church in Eastern Europe had a similar history.