An Online Public Access Catalog or OPAC is a computerized online catalog of the materials held in a library. The library staff and the public can usually access it at several computer terminals within the library, or from home via the Internet. Since the mid-1980s, it has replaced the card catalog in most libraries. Since the mid-1990s, character-based OPAC interfaces are being replaced by Web-based interfaces.

The largest OPAC is the giant OCLC WorldCat database, which is a catalog of materials held at collaborating public and private libraries worldwide. WorldCat only shows whether a given item is owned by a library system. It does not break down information by branch library or show which copies are currently on loan to users. WorldCat users must connect to a library's own OPAC to see that information.

OPAC Vendors

• Koha - Open Source
• Innovative Interfaces
• Sirsi