Z39.50 is an international standard client–server, application layer communications protocol for searching and retrieving information from a database over a TCP/IP computer network. It is covered by ANSI/NISO standard Z39.50, and ISO standard 23950. The standard's maintenance agency is the Library of Congress.

Z39.50 is widely used[as of?] in library environments, often incorporated into integrated library systems and personal bibliographic reference software. Interlibrary catalogue searches for interlibrary loan are often implemented with Z39.50 queries.

Work on the Z39.50 protocol began in the 1970s, and led to successive versions in 1988, 1992, 1995 and 2003. The Contextual Query Language (formerly called the Common Query Language)[1] is based on Z39.50 semantics.