Today we are witnessing an exponential growth of information accumulated within universities, corporations, and government organizations. Autonomous repositories that store different types of digital data in multiple formats are becoming available for use on the fast-evolving global information systems infrastructure. More concretely, with the World Wide Web and related internetworking technologies, there has been an explosion in the types, availability, and volume of data accessible to a global information system. However, this information overload makes it nearly impossible for users to be aware of the locations, organization or structures, query languages, and semantics of the information in various repositories. Available browsing and navigation tools assist users in locating information resources on the Internet. However, there is a real need to complement current browsing and keyword-based techniques with concept-based approaches. An important next step should be to support queries that do not contain information describing location or manipulation of relevant resources. Ontology-Based Query Processing for Global Information Systems describes an initiative for enhancing query processing in a global information system. The following are some of the relevant features: Providing semantic descriptions of data repositories using ontologies; Dealing with different vocabularies so that users are not forced to use a common one; Defining a strategy that permits the incremental enrichment of answers by visiting new ontologies; Managing imprecise answers and estimations of the incurred loss of information. In summary, technologies such as information brokerage, domain ontologies, and estimation of imprecision in answers based on vocabulary heterogeneity have been synthesized with Internet computing, representing an advance in developing semantics-based information access on the Web. Theoretical results are complemented by the presentation of a prototype that implements the main ideas presented in this book. Ontology-Based Query Processing for Global Information Systems is suitable as a secondary text for a graduate-level course, and as a reference for researchers and practitioners in industry.