When Pope Urban II launched the First Crusade in 1095, the aim was to capture Jerusalem which was then under Muslim rule. But before the crusade armies could reach the holy city, they would have to pass through the territory of Byzantium, a Christian empire centred on Constantinople. The Byzantine emperor, Alexios I, might have been expected to welcome the crusaders who were his co-religionists and who were on their way to fight his enemies, the Seljuk Turks. The relationship was in fact much more complicated thanks to diverging religious practices and very different political and ideological priorities. This lecture examines the taut relationship and the tragic consequences.