This book constitutes the refereed proceedings of the Third International Workshop on Rules and Rule Markup Languages for the Semantic Web, RuleML 2004, held in Hiroshima, Japan, in November 2004, together with ISWC 2004. The 11 revised full papers presented together with 2 invited papers and 5 tool presentation abstracts were carefully reviewed and selected from 25 submissions. Among the topics addressed are nonmonotonic rule systems, rule learning for feature extraction, logic reasoners for the Semantic Web, deductive RDF rule languages, description logic programs, defeasible description logics, conceptual logic programs, OWL inferencing, and Semantic Web reasoning.