Seymour de Ricci’s Census of Medieval and Renaissance Manuscripts in the United States and Canada, published by the Library of Congress in two volumes in 1935 and 1937 respectively, is a fundamental reference for the history of manuscripts in North American institutions and collections. This article explores the history of the Census' production, in particular the challenges that De Ricci faced in completing the monumental task of creating a union catalogue of manuscripts before the dawn of the digital age.