The Orthodox Church, the third main Christian faith after Catholics and Protestants, counts between 125 and 180 million believers around the world, mainly in Russia and eastern Europe.
After centuries of theological and political feuding, the schism between the eastern and western Christian churches came in 1054.
It started with the excommunication of the Patriarch of Constantinople, Michael Cerularius, who refused to recognise the primacy of the pope.
The papal envoy, Cardinal Humbert of Silva Candida, who ordered the expulsion, was excommunicated in his turn by Cerularius.
The two churches only lifted the excommunications in 1965.
The crusades, feared by most of the eastern Christians, aggravated the separation as the crusaders established Latin patriarchates alongside Greek patriarchates.
The word orthodox comes from the Greek "ortho" (right) and "doxa" (doctrine). The Orthodox Church claims possession of the "right faith" and considers that all other churches, including the Catholic Church, should join it.
One of the main differences between Catholic and Orthodox churches is over the Holy Spirit, which the Orthodox Church believes proceeds from God the Father, while for Catholics he proceeds from the Father and the Son.
The Orthodox faith also rejects Catholic dogma concerning the Immaculate Conception (under which the Virgin Mary is free from original sin), Mary's Assumption into Heaven, and the infallibility of the pope.
Unlike Catholics, Orthodox believers accept the ordination of married men, as well as divorce in the case of adultery.
While Catholic liturgy has developed over time, Orthodox liturgy has hardly changed since the first millenium.
Orthodoxy is organised into 14 patriarchates or independent churches, otherwise known as "autocephalic".
The four original patriarchates are those of Constantinople, Alexandria, Antioch and Jerusalem.
Ten independent churches were added over the centuries, which recognise the primacy of the patriarch of Constantinople but can themselves choose their own primates.
Four of these churches carry the title of patriarchate: Moscow, Serbia, Romania and Bulgaria. The others are the churches of Georgia, Greece, Cyprus, Albania, Poland and the church of the Czech Republic and Slovakia.
Some Orthodox churches recognise a fifteenth independent church: the Orthodox Church of America, based in New York.
Two other Orthodox churches enjoy autonomy, without being autocephalic: Finland, which is attached to the patriarchate of Constantinople, and Japan, linked to the patriarchate of Moscow.
The patriarchate of Kiev, self-proclaimed in 1992 after Ukraine's independence, has to this date not been recognised by any Orthodox church in the world. Several million Ukrainian Orthodox worshippers continue to be linked to the Moscow patriarchate.
The Patriarch of Constantinople -- currently Bartholomew I - has honorary and historical primacy over the other patriarchs of the Orthodox world.
He is considered to be first among equals: he does not have the right to intervene in the religious affairs of other Orthodox churches, but has a spiritual and official precedence.
The religious role of Constantinople, modern-day Istanbul, dates back to the Emperor Constantine who in 313 AD made Christianity the official religion of the Byzantine Empire and of its capital Constantinople.
Over the centuries, however, the patriarchate of Moscow, which has more believers than that of Constantinople, has taken the upper hand in the Orthodox world. Current Russian Patriarch Kiril is an ally of President Vladimir Putin.