This book constitutes the refereed proceedings of the First European Semantic Web Symposium, ESWS 2004, held in Heraklion, Crete, Greece in May 2004.The 33 revised full papers presented were carefully reviewed and selected from 79 submissions. The papers are organized in topical sections on ontology engineering, ontology matching and mapping, ontology-based querying, ontology merging and population, infrastructure, semantic web services, service discovery and composition, data from the semantic web, knowledge presentation, applications, content management, and information management and integration.