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The Library of Congress Control Number or LCCN is a serially based system of numbering cataloging records in the Library of Congress in the United States. This numbering system has been in use since 1898. It has nothing to do with the contents of any book, and should not be confused with Library of Congress Classification.

The acronym LCCN originally stood for Library of Congress Card Number. The Library of Congress prepared cards of bibliographic information for their library catalog and would sell duplicate sets of the cards to other libraries for use in their catalogs. This is known as centralized cataloging. Each set of cards was given a serial number to help identify it. As most of the bibliographic information is now electronically created, stored and shared with other libraries, there is still a need to identify each unique record, and the LCCN yet performs that function.

In its most elementary form the number includes a year and a serial number. The year has two digits for 1898 to 2000, and four digits beginning in 2001. The three ambiguous years are distinguished by the size of the serial number. There are also some peculiarities in numbers beginning with a "7" because of an unsuccessful experiment applied between 1969 and 1972.

Serial numbers are six digits long and should include leading zeros. The hyphen that is often seen separating the year and serial number is optional. More recently, the Library of Congress has instructed publishers not to include a hyphen.

Librarians all over the world use this unique identifier in the process of cataloging most books which have been published in the United States. It helps them reach the correct cataloging data (known as a cataloging record), which the Library of Congress and third parties make available on the Web and through other media.

• ISBN (International Standard Book Number)
• CODEN

• Structure of the LC Control Number
• Search the catalog to look up an LCCN