This article studies the role played by epistles in the Ecclesiastical History of Eusebius of Caesarea. It is important to note that this history avoids attributing discourses to the persons it features and instead relies on citing documents of various types, epistles among them. Having noted that the epistle is a special type of sermo, I discount the theory that the reason for the inclusion of the letters in the Ecclesiastical History was the desire to introduce stylistic variation. I emphasize that certain terms that recur in the introductions to the letters throw light on the question; an analysis of these words shows that Eusebius was very conscious of the importance that the copies of the texts that he cites as documents (letters, among others) be faithful to the literality of the originals. I propose as a hypothesis that this emphasis on the fidelity and literality of the documents is related to the scripture-related category of canonicity first attested in Eusebius's work