Before the advent of the Library of Congress's online catalog, researchers identified items in the Library's collections through the Library of Congress card catalog. The card catalog was a central tool located in the Library's Main Reading Room and adjacent spaces for most of the 20th century. It remains publicly accessible through two decks adjacent to the Main Reading Room.

The card catalog is an object of significant historical and cultural interest to researchers old and new. It is more than an artifact, however: the card catalog retains immense practical value when searching for books published through 1980, the final year new cards were added. The retrospective conversion of the card catalog to online form, performed by a private company, was often not as complete or accurate as desired. Thousands of catalog cards were never converted; records for many of the converted cards do not include the full information recorded on the original cards (e.g., subtitles, content notes, and series statements); and numerous typographical errors and misspellings during the conversion process can still prevent some records from being found in the online catalog through a keyword search. These and other issues make searching the card catalog an often necessary step for researchers conducting an exhaustive search for older books and periodicals held by the Library.

This guide discusses the origins and history of the Library's monumental card catalog, and also describes how researchers can continue to use it productively to search for books and periodicals published through 1980.