American National Information Standards Organization Z39.50 is the most popular synchronous search and retrieval protocol used in the library world. It is based on a client/server architecture and supports MARC format structures. Most integrated library systems offer options for both a Z39.50 client and a server package linked to their library OPAC infrastructure.

The Z39.50 maintenance agency is the Library of Congress, which also offers:

When implementing the protocol on a system, profiles must be defined and supported. The most popular and simple profile is the Bath profile and is available in English and French.

Further background information may be found on the following sites: