Erzbischöfliche Diözesan- und Dombibliothek Köln

The Cathedral Library traces back to the manuscript collection of Archbishop Hildebald (d. 818) and is considered the most significant among surviving cathedral libraries. The oldest catalogue, from 833, lists 175 books — 35 of which still exist today, with the oldest volume dating from 590/604. In the Middle Ages the cathedral chapter became its sole trustee, and many canons donated manuscripts; in the 11th century the capitular Hillinus gave the Hillinus Gospel Book, which bears an image of the old cathedral. In 1794, 200 of the most precious manuscripts were rescued from the French Revolutionary army and taken to Arnsberg together with the Shrine of the Three Kings; the Prussian state did not return the library to the cathedral chapter until 1867.

A second strand is the Diocesan Library, refounded in 1738 on the book collection of patron Johann Jakob von Broich — itself drawn from the estate of Jean-Paul Bignon, librarian to Louis XIV. Its holdings include art-historically significant choir books such as the Valkenburg Graduale. The collection grew through donations, bequests, and dissolved parish libraries, and in 1835 was joined by the 14,000-volume collection of Cologne Archbishop August Ferdinand Graf Spiegel, now housed in the Maternushaus.

Since 1930 the Diocesan and Cathedral Libraries have been organisationally united, with the Cathedral Library integrated as a permanent loan from the cathedral chapter. The Baroque library building on Marzellenstraße was completely destroyed near the end of the Second World War, though the holdings had been evacuated in time. In 1957 the collections were reunited in a new building on Gereonstraße; in 1983 the library moved to its current home in the Maternushaus.

In 2000 the library launched the Codices Electronici Ecclesiae Coloniensis (CEEC) project in partnership with the University of Cologne, digitising its medieval codices and becoming the world's first library to open up its medieval manuscripts in this way. Every two years it also hosts an interdisciplinary symposium dedicated to the study of its medieval holdings.