This article examines the Turkish threat to Byzantium in the mid-14th century, as seen through the eyes of a critical Byzantine intellectual and political figure of his time, Demetrios Cydones. Cydones was a consistent supporter of a resistance policy against the Ottoman Turks and strongly opposed any rapprochement. Having served three emperors from the 1340s to the 1380s, he sought a military alliance with western rulers through the Roman Catholic Church’s mediation. Cydones’ correspondence, which contains over 450 letters covering the years 1345-1391, is an essential source for the histories of Byzantium and the Ottoman Emirate in the second half of the 14th century. This article examines the Turkish threat to Byzantium through his letters. The letters contain accounts of Byzantine alliances with the Turks, the Ottoman conquest of Gallipoli, the first Turkish attacks on Constantinople and Thessaloniki, and the battle of Maritsa.