RELAX NG is an XML schema language developed by the RELAX NG technical committee at OASIS and designed to be an easy-to-learn and easy-to-implement alternative to the W3C's XML schema language. After several months of development and refinement, the RELAX NG TC has released an official committee specification for the language, along with a tutorial and DTD compatibility document.

RELAX NG was developed under the guidance of James Clark and MURATA Makoto (creator of the original RELAX schema language) and the members of the RELAX NG technical committee and already has an active and enthusiastic following, as evidenced by the large number of existing tools that implement the language.

Some of the features that developers and users are likely to find appealing in RELAX NG include:

James Clark has described the language in general as a "conservative, evolutionary refinement of well-proven ideas from SGML and XML DTDs". Readers interested in Clark's personal perspective on the language should read his Design of RELAX NG document.