This volume gathers together 277 letters of Pope Honorius III (1216-1227) concerning Frankish Greece and Constantinople. These letters constitute an indispensable source for the early history of the territories conquered during and just after the Fourth Crusade of 1204, for which almost no local archival material survives. The Latin texts of many of the letters are published here for the first time, and almost all the letters have been reedited from the manuscripts, primarily the papal registers in the Vatican Archives. In addition, the volume makes the letters available to non-specialists through exhaustive English summaries of all the letters and complete translations of the most significant ones. A lengthy historical introduction uses these letters to portray the dynamic world of the Latin Empire of Constantinople, the Kingdom of Thessaloniki, and the other states that replaced Byzantium, as the precarious condition of the Latin states compelled the ecclesiastical authorities in Rome to temper their ambitions of transcultural religious unity with pragmatic measures. It explores how this mixture of cultural idealism, practical necessity, and divergent class structures manifested themselves in Honorius’ policy towards the lower Greek clergy and Greek and Latin religious orders. Maps, tables, indices, and a guide to papal letters make the volume a useful tool for future studies of this fascinating and controversial phase in the history of Greece and the papacy.