The Library's collection contains approximately 400.000 volumes. About 60.000 of these continue to be located in the so-called Salone Monumentale; the rest of the collection is distributed in various storage areas throughout the Institute. The Library's patrimony is currently being increased by the acquisition of antiquarian materials as well as new publications, books from endowments, which enrich the Casanatense collections, theological texts, religious science (science or philosophy of religion) and medieval studies.