This book constitutes the refereed proceedings of the 3rd European Semantic Web Conference, ESWC 2006. The book presents 48 revised full papers with abstracts of 3 invited talks. The papers are organized in topical sections on ontology alignment, engineering, evaluation, evolution and learning, rules and reasoning, searching and querying, semantic annotation, semantic web mining and personalisation, semantic web services, semantic wiki and blogging, as well as trust and policies.