A unique source in many respects, the Codex epistolaris Carolinus comprises ninety-nine papal letters that were sent to the Carolingian court between 739 and 790. These are mostly addressed to the Frankish rulers Charles Martel, Pippin III, Carloman and Charlemagne. But there are also three letters grouped together in the collection about Adoptionism sent by Pope Hadrian I to the Spanish bishops. The letters shed light on many aspects of the burgeoning Frankish-papal relations in this period. They are therefore indispensable pieces of information on the history of the Franks, Lombards and Rome in the second half of the eighth century. This chapter offers some considerations with regard to its emergence in the episcopal library of Cologne. This chapter briefly addresses the historical background of the creation of the Codex Carolinus in the later eighth century and discusses the re-emergence of the Codex Carolinus in the later ninth century.