Type of site
|Network of library content and services|
|Available in||13 languages |
|Owner||Online Computer Library Center|
|Alexa rank||3,433 (February 2019 [update]) |
|Registration||Optional, but some features require registration (such as writing reviews and making lists or bibliographies)|
|Launched||January 21, 1998|
WorldCat is a union catalog that itemizes the collections of 72,000 libraries in 170 countries and territories  that participate in the Online Computer Library Center (OCLC) global cooperative. It is operated by OCLC Online Computer Library Center, Inc.  The subscribing member libraries collectively maintain WorldCat's database, the world's largest bibliographic database. OCLC makes WorldCat itself available free to libraries, but the catalog is the foundation for other subscription OCLC services (such as resource sharing and collection management).
OCLC was founded in 1967 under the leadership of Fred Kilgour.  That same year, OCLC began to develop the union catalog technology that would later evolve into WorldCat; the first catalog records were added in 1971.  
In 2003, OCLC began the "Open WorldCat" pilot program, making abbreviated records from a subset of WorldCat available to partner web sites and booksellers, to increase the accessibility of its subscribing member libraries' collections. In 2006, it became possible to search WorldCat directly at its website. In 2007, WorldCat Identities began providing pages for 20 million "identities", predominantly authors and persons who are the subjects of published titles. 
In December 2017, WorldCat contained over 400 million bibliographic records in 491 languages, representing over 2.6 billion physical and digital library assets,  and the WorldCat persons dataset ( mined from WorldCat) included over 100 million people. 
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WorldCat operates on a batch processing model rather than a real-time model. That is, WorldCat records are synchronized at intermittent intervals with the underlying library catalogs instead of real-time or every day. Consequently:
- WorldCat shows that a particular item is owned by a particular library but does not provide that library's call number.
- WorldCat does not indicate whether or not an item is currently borrowed, lost, undergoing restoration or repair, or moved to storage not directly accessible to patrons (thereby forcing interested patrons to submit a retrieval request and wait).
- Furthermore, WorldCat does not show whether or not a library owns multiple copies of a particular title.
As an alternative, WorldCat allows participating institutions to add direct links from WorldCat to their own catalog entries for a particular item, which enables the user to determine its real-time status.  However, this still requires users to open multiple Web pages, each pointing to a different online public access catalog with its own distinctive user interface design (which places item status in a different portion of the Web browser display), until they can locate a catalog entry that shows the item is currently available at a particular library.
- Faceted Application of Subject Terminology (FAST)
- Library and Archives Canada
- Open Library
- Research Libraries UK
- "Search for library items". WorldCat. Online Computer Library Center. Retrieved March 29, 2017.
- "Worldcat.org Traffic, Demographics and Competitors – Alexa". www.alexa.com. Retrieved February 4, 2019.
- "WorldCat.org WHOIS, DNS, & Domain Info – DomainTools". WHOIS. Retrieved January 21, 2017.
- "Inside WorldCat". Online Computer Library Center. Retrieved February 11, 2018.
- "What is WorldCat?". worldcat.org. Retrieved February 13, 2015.
Margalit Fox (August 2, 2006).
"Frederick G. Kilgour, Innovative Librarian, Dies at 92".
The New York Times. Retrieved December 22, 2009.
Frederick G. Kilgour, a distinguished librarian who nearly 40 years ago transformed a consortium of Ohio libraries into what is now the largest library cooperative in the world, making the catalogs of thousands of libraries around the globe instantly accessible to far-flung patrons, died on Monday in Chapel Hill, N.C. He was 92.
- "A brief history of WorldCat". oclc.org. February 10, 2015. Retrieved February 13, 2014.
- Hickey, Thomas B. (April 15, 2007). "WorldCat Identities: Another View of the Catalog" (PDF). NextSpace. OCLC (6): 18–19. ISSN 1559-0011. Retrieved January 18, 2016.
- "Data strategy [WorldCat]". oclc.org. Retrieved February 11, 2018.
- Blackman, Cathy; Moore, Erica Rae; Seikel, Michele; Smith, Mandi (July 2014). "WorldCat and SkyRiver: a comparison of record quantity and fullness". Library Resources & Technical Services. 58 (3): 178–186. doi: 10.5860/lrts.58n3.178.
- Breeding, Marshall (May 2015). "Library services platforms: a maturing genre of products". Library Technology Reports. 51 (4): 1–38. doi: 10.5860/ltr.51n4.
- Matthews, Joseph R. (July 2016). "An environmental scan of OCLC alternatives: a management perspective". Public Library Quarterly. 35 (3): 175–187. doi: 10.1080/01616846.2016.1210440.
- McKenzie, Elizabeth (January 2012). OCLC changes its rules for use of records in WorldCat: library community pushback through blogs and cultures of resistance (Technical report). Boston: Suffolk University Law School. Research paper 12-06.
- What the OCLC online union catalog means to me: a collection of essays. Dublin, Ohio: OCLC. 1997. ISBN 1556532237. OCLC 37492023.
- Wilson, Kristen (August 2016). "The knowledge base at the center of the universe". Library Technology Reports. 52 (6): 1–35. doi: 10.5860/ltr.52n6.
- "WorldCat data licensing" (PDF). oclc.org. Retrieved December 31, 2018. See also: "Data licenses & attribution". oclc.org. Retrieved December 31, 2018. Information about licensing of WorldCat records and some other OCLC data.