This study is an account of the various ways in which relations between the Byzantine Empire and the West developed during the reign of John V Ralaiologos, 1341-1391. There is an extensive literature (of which a selection is given in the bibliography) on special aspects of the subjects) the thesis completes and corrects this literature on several points and draws the various aspects together in a manner not previously attempted. After an introductory chapter describing the problems which faced John V at his accession, emphasising the significance of the great losses of territory sustained by the empire between 1341 and 1354 and outlining the importance of the West in John V's foreign policy, there are three main sections dealing with i) The formal negotiations concerning the reunion of the Churches of Constantinople and Rome, ii) The individual and intellectual contacts between Bast and West, iii) The commercial relations. There is also an Appendix dealing with the problem of the coinage of Constantinople during the period - a problem which throws light upon the financial position of the empire in relation to the West. The substance of these sections may be summarised as follows. i) During the reign of John V the problem of obtaining western military aid for Byzantium was closely linked to the question of the reunion of the Churches of East and West.