Since the publication of the first edition of The Description Logic Handbook in 2003, the interest in Description Logics (DL) has steadily increased. This applies both to the number of active DL researchers working on DL theory and implementations of reasoning services, and to the number of applications based on DL technology. One effect of this growing interest was that the first edition of the Handbook has gone through quite a number of reprints. Another effect is, of course, that in the last three years there have been interesting new developments in the three areas (theory, implementation, and applications) that the Handbook covers. Despite that, we feel that most chapters of the Handbook still provide a good introduction to the field and lay a solid foundation that enables the reader to understand and put into context the research articles describing results since 2003. For this reason, we have decided to leave most of the chapters unchanged.

The principal exception is Chapter 14, which in the first edition was entitled “Digital Libraries and Web-Based Information Systems.” This chapter provided a selected history of the use of Description Logics in web-based information systems, and the developments related to emerging web ontology languages such as OIL and DAML+OIL. Since the writing of this chapter, the new language OWL has been developed and recommended by the World Wide Web consortium as the standard web ontology language for the Semantic Web. In the second edition, Chapter 14, now co-authored by Peter Patel-Schneider, concentrates on OWL, which is reflected by its new title: “OWL: a Description-Logic-Based Ontology Language for the Semantic Web.” The chapter still briefly reviews some early efforts that combine Description Logics and theWeb, including predecessors of OWL such as OIL and DAML+OIL. But then it goes on to describe OWL in some detail, including the various influences on its design, its relationship with RDFS, its syntax and semantics, and a range of tools and applications.