This work is a revision of the doctoral dissertation of Eduardo Mena pre- sented to the Department of Computer Science and System Engineering at the University of Zaragoza (Spain) in November 1998 Mena 98]. The OBSERVER system was developed as a result of this Ph.D. thesis. This book is composed of eight chapters. In Chapter 1 we introduce our rationale for writing a book about systems that process queries in global information systems. Then in Chapter 2 we review the techno- logical context for our work, including distributed and heterogeneous environments and the use of ontologies. We also...