This book constitutes the refereed proceedings of the 6th European Semantic Web Conference, ESWC 2009, held in Heraklion, Crete, Greece, in May/June 2009. The 45 revised full papers of the research track presented together with the abstracts of 4 keynote lectures were carefully reviewed and selected from more than 250 submissions. The papers are organized in topical sections on applications, evaluation and benchmarking, ontologies and natural language, ontology alignment, ontology engineering, query processing, reasoning, search and identities, semantic Web architectures, semantic Web services, and tagging and annotation. In addition to the technical research track, this book presents 8 contributions to the ESWC 2009 PhD symposium, 24 system demo papers, as well as 8 contributions to the semantic Web in-use track.