In early seventeenth-century Europe it was considered the most widely published work next to the Bible: The Imitation of Christ easily ranks as one of the most translated, printed, and circulated works of early modern Europe among Catholic, Protestant, and even some Orthodox confessional circles, yet until the present, the medieval origins of this work have received little attention. Yet it would seem to have been every bit as popular from soon after it first appeared in the early 1420s. Widely circulated in manuscript over the next half-century, more than eight-hundred medieval manuscripts survive to the present day, with most made between 1424 and 1480 when printed copies began to circulate widely. This dissertation examines the manuscripts of this work as evidence of its circulation as a mass communications phenomenon and an indicator of the religious culture of fifteenth-century reform groups. Written as a personal spiritual exercise by Thomas of Kempen, an obscure canon regular from the Netherlands, the work's earliest dated manuscripts bear witness to a relatively limited circulation prior to 1435. After that year, manuscripts survive at nearly three times the rate of the previous ten years, a trend that continues for the next four decades. This rapid increase in circulation coincides with the Council of Basel (1433-1438) which facilitated increased exchange of information thanks to the communications networks and public forums which grew up in response to the Council. In addition to changes in the quantity of circulation, the nature of the work's reception changed after the 1430s. In the later period, the work expanded geographically from its earliest circle in the Northwest German lands to nearly every region of Europe. The earliest dated copies from Austria, Bavaria, and France (1435), Italy (1436), and England (1438) all date from this period. And as the work's geographic diffusion increased, its circulation began to exhibit multiple centers or "distribution circles'' where the work spread in forms uniquely suited to the needs of the communities that copied the work. It was this multiple agency and adaptability in the manuscript cultures of fifteenth-century Europe that provided a precedent for the adaptation of this work among the circles and centers of a confessionally divided age, and continue to explain its enduring appeal to diverse audiences even today.