The Codex epistolaris Carolinus is a remarkable source for the history of the Franks, Lombards and Rome in the eighth century. It is a compilation of ninety-nine letters from popes Gregory III, Zacharias, Stephen II, Paul I, the anti-pope Constantine, Stephen III and Hadrian I to, respectively Charles Martel, Pippin III, Carloman and Charlemagne, with three letters also sent by Hadrian I to Spain and one letter purporting to be from St Peter himself. The compilation was commissioned by Charlemagne in 791 and survives in a single manuscript, Codex Vindobonennsis 449, copied in the late ninth century and owned by Archbishop Willibert of Cologne (870-89). The letters address a great variety of topics, such as the politics of Italy, the popes' need for support in relation to the Lombards, the popes' territorial claims, sending gifts and advice to the Frankish rulers, commenting on aspects of canon law, expounding Old Testament parallels for the Frankish rulers to emulate, and protesting vigorously against any indication of the Carolingian rulers allying with the Lombard kings. The letters between Charlemagne and Hadrian in particular reveal the strength of the relationship established between the two rulers. The less well known set of letters to Pippin, especially the letters from Paul I to Pippin is an extraordinarily important source of information about the politics of the Lombard kingdom in the reigns of Aistulf and Desiderius in particular, and on politics in Rome, with reports of papal elections and disputes. This is the first complete translation of all the letters; together with the substantial introduction it will facilitate the appreciation of the significance and political role of the collection as a whole.