The Eastern Orthodox claim a direct, unbroken descent from the church of the apostles. This is expressed in their fidelity to the apostolic faith as developed in the seven ecumenical councils and the patristic tradition. Thus, the Eastern Orthodox churches are united in the faith, and each one has internal autonomy under the primacy of the patriarchate of Constantinople, the "first among equals".

Orthodoxy also implies a strong attachment to the sacraments, the most important being the sacraments of initiation: baptism (by immersion), chrismation and the Eucharist (communion in both kinds), to which the newly baptized member is immediately admitted, whatever his or her age.

Since the separation from the Christian West, Eastern Orthodox churches have mainly been using the Syro-Byzantine liturgical tradition , whose development owes much to the fathers and the great monastic centres (today, Mt Athos is the most important of these). In this liturgical tradition iconography plays an important part.

Structurally, Eastern Orthodox churches currently fall under the following classifications. They represent four out of the five ancient patriarchates which, together with Rome, formed the famous pentarchy, i.e. Constantinople (Patriarch Bartholomew I; some 2 million faithful, with only a few thousand in Turkey), Alexandria (Patriarch Peter VII, about 100,000 faithful), Antioch (primatial see: Damascus; Patriarch Ignatius IV; some 450,000 faithful) and Jerusalem (Patriarch Diodorus; about 50,000).

Orthodoxy includes a number of other autocephalous churches (i.e. churches that elect their own primate without reference to another autocephalous church). The largest of all is the church of Russia (Patriarch Alexis II; about 100 million faithful in 1917; today, probably over 80 million). Others are the Romanian church (Patriarch Theoctist; some 14 million); the Serbian church in Yugoslavia (patriarchate in Belgrade; Patriarch Pavle; some 8 million); the church of Greece, distinct from the patriarchate of Constantinople since 1833, with its own primate, the archbishop of Athens (Seraphim) (about 7.5 million faithful); the Bulgarian church (Patriarch Maximos; some 6 million); the church of Georgia, a second autocephalous church on ex-Soviet territory, much more ancient than the Russian church (founded in the 5th century as a result of missionary work by a woman, St Nino, counted as "equal to the apostles" in the Orthodox sanctoral; Patriarch Catholicos Elias II; 2.5 million faithful in 1917); the church of Cyprus (autocephalous since the council of Ephesus, 431; Archbishop Chrysostomos; some 450,000 faithful).

A third type are autocephalous churches which represent a minority among other Christians in their territory, namely, the Orthodox church of Czechoslovakia (some 350,000 faithful in 1950); the Orthodox church of Poland (about 350,000); the Orthodox church of Albania (about 210,000 faithful in 1944), now developing once again under the leadership of Archbishop Anasthasios (Yannoulatos).

Orthodoxy also includes autonomous or semi-autonomous churches (i.e. churches that enjoy internal autonomy but whose primate is elected under the aegis of one of the autocephalous churches). Among them are the church of Finland (some 70,000 faithful, under the jurisdiction of Constantinople); the church of Crete (also under Constantinople); the Orthodox church of Japan (about 36,000 faithful, under the jurisdiction of Moscow); the Russian Orthodox mission in China (probably some 20,000 faithful).

Another classification is missions which are not yet autonomous. These include the Russian mission in Korea (under the jurisdiction of the Greek Archdiocese of North America) and African Orthodoxy (founded in Uganda by dissidents from Anglicanism, now present also in Kenya, Zaire and Ghana, under the jurisdiction of the patriarchate of Alexandria).

Finally, there is the Orthodox diaspora. In the 19th and 20th centuries, many Orthodox emigrated to Western countries for economic and political reasons. As a result, Orthodox are now to be found in most parts of the world.

Although the principle of identifying Orthodoxy with an ethnic group was condemned as a heresy in 1872 under the name of "phyletism" by a synod held in Constantinople (but received by all Orthodox churches), the present situation resembles a complicated jigsaw puzzle of numerous jurisdictions in most countries of the Western world, where the Orthodox of various origins tend to be claimed by their mother churches according to their ethnicity.

According to traditional Orthodox ecclesiology, all the Orthodox in a given place, whatever their ethnic origin, should be gathered in one conciliar communion. Such, for example, was the situation in the US until 1917: all the Orthodox were in one diocese (which had grown from the Russian mission among the Aleutian and Alaskan Indians in the 18th century). At the council of Moscow in 1917, Tikhon, the bishop of the American diocese (recently canonized), was elected patriarch. When he was able to send a successor to New York a few years later, the latter found that in the meantime all the mother churches of the Orthodox world had claimed their nationals and created their own jurisdictions. In 1970, the Russian church granted autocephaly to the churches of its old diocese in America, thus creating the Orthodox Church in America (primate: Metropolitan Theodosius). However, finding the solution to the problem of the Orthodox diaspora remains one of the main difficulties of present-day Orthodoxy, one that is high on the agenda of the pan-Orthodox council. Recently the churches have moved towards a consensus in this area.

Eastern Orthodox churches have played a part in the ecumenical movement from an early date in the 20th century. Witness the encyclical letter of the ecumenical patriarch of Constantinople in 1920 to "all the churches of Christ" for "closer intercourse and mutual co-operation". The Orthodox diaspora has also greatly contributed to an encounter with Western Christendom, to better mutual understanding, and to a common renaissance in patristic theological reflection. Most Eastern Orthodox churches have become members of the WCC and have established bilateral dialogues with most Christian churches. Orthodoxy, however, does include a certain anti-ecumenical strain which is largely due to a suspicion on the part of some that ecumenical dialogue necessarily implies a betrayal of the purity of the Orthodox faith.

Eastern Orthodox churches do not believe in "intercommunion"; in their view, only full communion has a meaning. This is the main reason why the Orthodox generally have refused to practise so-called eucharistic hospitality. In their conception of the nature of the church, communion is only possible when the apostolic faith can be fully confessed together. (Some pastors do practise eucharistic hospitality in specific circumstances, but only as a matter of conscience in their personal pastoral responsibility.) For the time being, Eastern Orthodox churches are not prepared to sanction a generalized eucharistic hospitality, not even as a measure of economy. Indeed, such a step would amount to establishing a rule, and the principle of economy is precisely a pedagogical exception to a rule which in no way abolishes the existing rule. In the Orthodox perspective, full communion will quite naturally be restored when it is truly possible to confess the fullness of the apostolic faith together.