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We use formal ontologies to represent knowledge about digital library content and services. Formal ontologies define concepts with logic in a frameinheritance structure. The expressiveness and precision of these structures supports computational reasoning that can be used in important ways. This paper focuses on the creation of ontological metadata.We create ontological content metadata by generating it from MARC (MAchine Readable Cataloging) data. MARC contains much information that is hard to exploit computationally. In particular, relationships between works are implicit in shared values and natural language notes. The conversion process involves specifying an ontological model, mapping MARC to the ontology, and reasoning about the data to create explicit links between works.Service metadata will be supplied by providers who wish to participate fully in a digital library that is implemented as a decentralized multi-agent system. Agents advertise by describing their services in terms of ontologically defined concepts. We reason about these descriptions to organize them into subsumption taxonomies. Agents can then find the best available services to meet their needs by describing their needs, without requiring a priori knowledge of other agents. This infrastructure has demonstrated its usefulness in a multi-agent system organized as a computational economy.

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We propose a new catalog based on a formal ontological model of bibliographic relations. A hierarchy of five central concepts describes the creation of work. Each kind of relation between works occurs at a particular level in the hierarchy. Related works share data at some level of the hierarchy, yielding a tree structure that reduces redundant representation of shared attributes.To show that ontology-based metadata is practical, we generated a knowledge base of metadata from a sample of MARC records. We implemented the ontology in description logic (Loom), mapped Machine Readable Cataloging (MARC) attributes and values to the ontology, and loaded the data into Loom with all values treated its separate instances. We then unified matching instances, and deduced relations between works. This process thus converts relationships implicit in MARC into explicit relations that arc easy to utilize with computers.Our web interface permits browsing by navigating relations between works. Ontology-based metadata can also support user inquiry and digital-library operation in other important ways.