After years of mostly theoretical research, Semantic Web Technologies are now emerging into application areas like bioinformatics, eCommerce, eGovernment and Social Webs. The Handbook of Semantic Web Technologies, compiled by three leading authorities in the field, and supported by an advisory board of highly reputed researchers, fulfils exactly this need. It is the first dedicated reference work in this field, collecting contributions about both the technical foundations of the Semantic Web as well as their main usage in other scientific fields like life sciences, engineering, business, or education.