In the late third century Eusebius of Caesarea, better remembered now for his work as a historian of the church, produced an apparatus for the reconciliation of the disagreements found in the four Christian gospels. It was a remarkable work in its own right for it preserved, as the tradition demanded, the plurality of the gospels, while allowing them to be presented and studied as a single entity, “the gospel,” and so succeeding in Tatian's aim in his Diatessaron — as exegesis and apologetics demanded. Moreover, though now largely forgotten, it remained an important element within theology for centuries. This paper's aim is to locate the significance of Eusebius's work in its original setting in the world of late antiquity and the Christian defense of pagan challenges to the gospels' integrity, and then to follow the influence of his work within just one strand of the tradition: that which forms the background of western, Latin theology. So it will note how that work was adopted and adapted by Jerome, how it then passed on to the late-patristic Latin schoolmasters who sought to transform all learning into convenient modules of defined value, and then was taken up by others in just one region of the Latin West, the insular world, such as the anonymous scribes of the Book of Kells, the Stowe Missal, and the Book of Deer, for whom Eusebius's work was a mystery that they could not simply abandon, even when they could not understand it. Throughout this period, the Eusebian Apparatus roused the intellect of scholars, teachers, and scribes, but in each milieu the significance and perceived utility of the Apparatus was different. The history of ideas is about changes within intellectual and textual continuities, and with the Apparatus we have a clearly identifiable scholarly tool that does not in itself change over the period, but whose reception and exploitation vary greatly.