The East–West Schism is the medieval division of Chalcedonian Christianity into Eastern (Greek) and Western (Latin) branches, which later become commonly known as the Eastern Orthodox Church and the Roman Catholic Church, respectively.

The East–West Schism is one of the two schisms to which the term "Great Schism" is applied (the other being the Western Schism).

Relations between East and West had long been embittered by ecclesiastical differences and theological disputes.

Prominent among these are the issues of the source of the Holy Spirit ("filioque"), whether leavened or unleavened bread should be used in the Eucharist, the Pope's claim to universal jurisdiction, and the place of Constantinople in relation to the Pentarchy.

In 1053, the first step is taken in the process which leads to formal schism.

Patriarch of Constantinople Michael Cerularius orders the closure of all Latin churches in Constantinople.

In 1054, the Papal legate travels to Constantinople for purposes that include refusing to Cerularius the title of "Ecumenical Patriarch" and insisting that he recognize Rome's claim to be the head and mother of the churches.

The main purpose of the papal legation is to seek help from the Byzantine Emperor in view of the Norman conquest of southern Italy and to deal with recent attacks by Leo of Ohrid against the use of unleavened bread and other Western customs, attacks that have the support of Cerularius (Michael Caerularius); Axel Bayer says the legation was sent in response to two letters, one from the Emperor seeking assistance in arranging a common military campaign by the eastern and western empires against the Normans, and the other from Cerularius.

On the refusal of Cerularius to accept the demand, the leader of the legation, Cardinal Humbert, excommunicates him, and in return Cerularius excommunicates Cardinal Humbert and the other legates.

This is only the first act in a centuries-long process that eventually becomes a complete schism.

The validity of the Western legates' act is doubtful, since Pope Leo had died and Cerularius' excommunication applied only to the legates personally.

Still, the Church split along doctrinal, theological, linguistic, political, and geographical lines, and the fundamental breach has never been healed, with each side sometimes accusing the other of having fallen into heresy and of having initiated the division.

The Crusades, the Massacre of the Latins in 1182, the West's retaliation in the Sacking of Thessalonica in 1185, the capture and sack of Constantinople in 1204, and the imposition of Latin patriarchs make reconciliation more difficult.

Establishing Latin hierarchies in the Crusader states means that there are two rival claimants to each of the patriarchal sees of Antioch, Constantinople, and Jerusalem, making the existence of schism clear.

The Second Council of Lyon, in 1274, and the Council of Florence in 1439 attempts to reunite the two churches.

Despite acceptance by the participating eastern delegations, no effective reconciliation is realized, since the Orthodox believe that the acts of councils must be ratified by the wider Church and the acts of these councils never attain widespread acceptance among Orthodox churches.

In 1484, 31 years after the Fall of Constantinople to the Ottoman Turks, a Synod of Constantinople repudiates the Union of Florence, officially stating the position that had already been taken by Orthodox in general.In 1965, Pope Paul VI and the Ecumenical Patriarch of Constantinople Athenagoras I nullify the anathemas of 1054, although this nullification of measures taken against a few individuals is essentially a goodwill gesture and does not constitute any sort of reunion between churches.

Contacts between the two sides continue: Every year a delegation from each joins in the other's celebration of its patronal feast, Saints Peter and Paul (29 June) for Rome and Saint Andrew (30 November) for Constantinople, and there have been a number of visits by the head of each to the other.

The efforts of the Ecumenical Patriarchs towards reconciliation with the Catholic Church have often been the target of sharp criticism from some fellow Orthodox.