WorldCat is a union catalog which itemizes the collections of 72,000 libraries in 170 countries and territories which participate in the Online Computer Library Center (OCLC) global cooperative. It is built and maintained collectively by the participating libraries.

The catalog was created in 1971 and, as of April 2012^[update], it contains more than 264 million different records pointing to over 1.83 billion physical and digital assets in more than 470 languages. It is the world's largest bibliographic database. OCLC makes WorldCat itself available free to libraries, but the catalog is the foundation for other fee-based OCLC services (such as resource sharing and collection management). WorldCat was founded by Fred Kilgour in 1967.

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 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.