WorldCat is a union catalog that itemizes the collections of tens of thousands of institutions (mostly libraries), in many countries, that are current or past members of the OCLC global cooperative.

[10][11] In October 2005, the OCLC technical staff began a wiki project, WikiD, allowing readers to add commentary and structured-field information associated with any WorldCat record.

[13][14] In 2006, it became possible for anyone to search WorldCat directly at its open website WorldCat.org,[15] not only through the subscription FirstSearch interface where it had been available on the web to subscribing libraries for more than a decade before.

[23] In August 2022, OCLC launched a "redesigned and reimagined" WorldCat.org website with the stated goal "to offer greater accessibility to the collections".

[30] Library contributions to WorldCat are made via the Connexion computer program,[31] which was introduced in 2001; its predecessor, OCLC Passport, was phased out in May 2005.