The National Union Catalog (NUC) is a catalogue of books held by over 1,100 libraries in Canada and the United States. The NUC is composed of two major series: one that covers books published before 1956, and one that covers books published from 1956 onwards. During the mid- to late-19th century the increase in the number of universities and researchers resulted in an explosion of the number of books, journals, newspapers and other forms of communication. National bibliographies, such as the NUC, were an attempt to capture the intellectual output of a nation. The Library of Congress in Washington, D.C. began compiling the catalogue in 1901, with the goal of identifying every important book in the United States. As such the NUC became an essential resource for those looking for print materials.

By the early 1960s, the Library of Congress was issuing supplements to the catalogue on a monthly basis; in the face of its growing complexity, they decided to compile all pre-1956 imprints into a single collection. The resulting catalogue encompassed 754 volumes, contained over 528,000 pages, and weighed three tonnes. Its full title is National Union Catalog, Pre-1956 Imprints: A Cumulative Author List Representing Library of Congress Printed Cards and Titles Reported by Other American Libraries.

The National Union Catalog may be found printed in its entirety at the Library of Congress, though many other libraries (including U of T Libraries) hold a subset of the catalogue. While online union catalogues provide easy access to bibliographic information, studies have shown that some old and rare titles are only recorded in the print NUC. For this reason it remains an essential tool for scholars.

What is the National Union Catalog?