RELAX NG is a robust schema language for XML, designed for simplicity and ease of learning. Developed within OASIS by the RELAX NG Technical Committee and later standardized as ISO/IEC 19757-2, it offers both XML and compact non-XML syntaxes. Key features include uniform treatment of attributes and elements, unrestricted mixed content, and a solid theoretical base.

Its ability to partner with separate datatype languages like W3C XML Schema Datatypes adds flexibility. The specifications have evolved through OASIS Committee Specifications and ISO versions, with RELAX NG Version 2 likely to become part of the DSDL standard while Version 1 remains relevant. Tools like Jing (Java-based validator) and RNV (C validator) support validation, alongside converters like Trang for schema format conversion.

Books like Eric van der Vlist's work and online resources provide practical guidance. This foundation underpins XML validation across diverse applications, ensuring structured data integrity.