Library of Congress collections contain over 175 million books, periodicals, manuscripts, maps, music, recordings, images, and electronic resources. The LC Catalog contains over 20 million records describing these collections.  

Library of Congress cataloging dates back to 1898. The LC Catalog includes many early records (primarily for books and periodicals) created by the Library between 1898 and 1980. These records are gradually being updated to reflect contemporary language and usage. Authors/creators and subjects found on older records often contain the legend "[from old catalog]" to indicate possible deviations from current LC practices. In addition, records for some materials cataloged before 1980 are only available to researchers onsite in the Library's Main Card Catalog. 

Items in the Library's special collections are generally described as aggregations or archival collections rather than individually identifying each specific item, with records for many archival collections linked to more detailed guides available in Finding Aids Search. Some Library special format materials are also discoverable via separate expert search systems (such as the Prints and Photographs Online Catalog) that supplement search available from Library of Congress Home. Some records for digitized items may only be found on the Library's Digital Collections website. Databases, ejournals, and ebooks that are available to patrons onsite (including items found in fee-based online services) can be located through the E-Resources Online Catalog. For more information, see More Catalogs and Research Tools.