Although Christians have always condemned schism as contrary to the necessary unity of the Church, divisions have always marred the Church's life. One of the most significant is that between the Roman Catholic and Eastern Orthodox Churches. Their gradual doctrinal and liturgical divergence hardened into ecclesial hostility partly as a consequence of political developments. But even after the Fourth Crusade's capture of Constantinople in 1204 and the establishment of a Latin empire and patriarchate, both sides retained a conviction that schism should be overcome. From the thirteenth century onwards calls for a council to resolve differences and re-establish unity led to the Councils of Lyons in 1274 and of Ferrara-Florence in 1439. In both cases the anti-unionist party in the Orthodox Church successfully opposed the restoration of communion nominally achieved by the imposition of Roman doctrine, in particular the filioque and papal primacy, on an Orthodox Empire increasingly desperate for Western military help against the Ottoman Turks. The papers collected in this book, mostly in French, were read at a conference with the same title in 2011, and are grouped under three headings. In the first section the authors examine initiatives taken from the thirteenth to the eighteenth century to achieve reunion between Greeks and Latins, and the polemical context in which these initiatives took place. Those initiatives led to the councils of 1274 and 1439 and to the Union of Brest in 1595–6. The papers in the second section look at the doctrinal reasons advanced by Orthodox theologians, among them Gregory Palamas, to justify their opposition to union on the terms proposed by the Catholics, and imposed by Rome at both union councils. In the third section the authors examine relations between Greeks and Latins in areas where the two Churches were in close geographical contact. In the seventeenth and eighteenth centuries, although Rome encouraged vigorous missionary efforts to bring the Orthodox into its communion, relations between the two Churches were not always and everywhere hostile. There were even instances of local intercommunion. From the sixteenth century several groups of Orthodox entered into communion with Rome while maintaining their liturgical and other traditions, most notably in 1724, when a group within the Patriarchate of Antioch entered into union with Rome to form the Greek Catholic or Melkite Church. The existence of such ‘uniate’ Churches remains one of the obstacles in the way of re-establishing communion between Orthodox and Roman Catholics. Paradoxically, as one of the editors observes, it was perhaps discussions about the idea of a Union that helped to bring the schism into being, at least as a precisely defined, rigid reality. This book studies in specific detail how those discussions brought into sharper focus the disagreements that they sought to overcome, in the hope that greater understanding of the past might help to heal the schism in the present.