R2ML is developed by the REWERSE Working Group for the purpose of rules interchange between different systems and tools, enriching of ontologies with rules, connecting of custom rule system with R2ML-based tools for vizualization, verbalization, verification and validation. R2ML is comprehensive in the sense that it integrates the Object Constraint Language (OCL) - a standard used in information systems engineering and software engineering, the Semantic Web Rule Language (SWRL)- a proposal to extend the Semantic Web ontology language OWL by adding implication axioms, the Rule Markup Language (RuleML) - a proposal based on Datalog/Prolog, and it includes four rule categories: derivation rules, production rules, integrity rules and ECA/reaction rules. R2ML provides a rich syntax for expressing rules supporting conceptual distinctions between different types of terms and different types of atoms, which are not present in standard predicate logic.