Among the events that have shaped the history of Europe a notable one is the influence exerted by Byzantium on the cultural life of Eastern Europe, i.e., Bulgarians, Albanians, Serbs, Romanians, Ukranians, and Russians. One of the more prominent are the Russians. On two occasions, once in the eleventh and twelfth centuries, and again in the fourteenth and fifteenth, Byzantium exerted a formative influence on the society and culture of Rus'. Unless this influence is properly appreciated and understood, much in Russia's history will remain unintelligible.