This book constitutes the refereed proceedings of the Third International Semantic Web Conference, ISWC 2004, held in Hiroshima, Japan in November 2004. The 55 revised full papers presented together with abstracts of 2 invited talks were carefully reviewed and selected from a total of 227 submitted papers. The papers are organized in topical sections on data semantics, p2p systems, semantic Web mining, tools and methodologies for Web agents, user interfaces and visualization, large scale knowledge management, semantic Web services, inference, searching and querying, semantic Web middleware, integration and interoperability, ontologies, and industrial track.