This book constitutes the refereed proceedings of the 5th International Semantic Web Conference, ISWC 2006, held in Athens, GA, USA in November 2006. The 52 revised full academic papers and 14 revised application papers presented together with the abstracts of 3 invited talks and 12 selected doctoral consortium articles were carefully reviewed and selected from a total of 215 submitted papers to the academic track and 42 to the applications track. The research papers address all current issues in the field of the semantic Web, ranging from theoretical aspects to various applied topics. The application track "Semantic Web In Usea contains papers on applications in government, public health, public service, academic, and industrial source - such as new technologies for building applications, and methodological and feasibility aspects of building industrial applications that incorporate semantic Web technology. Short descriptions of the top five winning applications submitted to the Semantic Web Challenge competition conclude the volume.