ONIX is a group of related XML standards for books, serials and publishing rights information.

Onix for Books was the first of the standards to be widely adopted by the book trade and was developed by EDItEUR with Book Industry Communication (UK) and the Book Industry Study Group (US) and is currently maintained with the guidance of an International Steering Committee. The ONIX for Books Product Information Message is de facto international standard for the electronic communication book trade product information.

Onix for Books is a very comprehensive and sophisticated format, in order to allow as much functionality as possible in the different environments. The most widely implemented release, Onix 2.1 was the first truly international descriptive metadata format to be adopted by the book industry and its successor Onix 3.0, has been further enhanced for e-books.

Libraries have long been interested in the potential for using publisher information as a basis for catalogue records in order to improve efficiency.  Publisher migration from proprietary local formats to Onix has made this a more realistic proposition by reducing the overhead in maintaining multiple translations to MARC. NBAs responsible for maintaining a CIP programme often accept Onix formatted files as notification of forthcoming titles from publishers. Details of Onix to MARC 21 mappings created by OCLC and the Library of Congress can be found on the EDItEUR web site.