Under the appellation of “ontology” are found many different types of artifacts created and used in different communities to represent entities and their relationships for purposes including annotating datasets, supporting natural language understanding, integrating information sources, semantic interoperability and to serve as a background knowledge in various applications. The Ontology Summit 2007 “Ontology, taxonomy, folksonomy: Understanding the distinctions”,1 was an attempt to bring together various communities (computer scientists, information scientists, philosophers, domain experts) having a different understanding of what is an ontology, and to foster dialog and cooperation among these communities. In practice, ontologies cover a spectrum of useful artifacts, from formal upper-level ontologies expressed in first order logic, such as Basic Formal Ontology (BFO), Descriptive Ontology for Linguistic and Cognitive Engineering (DOLCE), Suggested Upper Merged Ontology (SUMO), and Process Specification Language (PSL), to folksonomies (the simple lists of user-defined keywords to annotate resources on the Web). In between these two extremities of the ontology spectrum are taxonomies, conceptual models and controlled vocabularies such as Medical Subject Headings (MeSH), often used for information indexing and retrieval, and whose organization is mostly hierarchical. Finally, there are ontologies which represent not only subsumption, but also other kinds of relationships among entities (e.g., functional, physical), often based on formalisms such as frames or description logics. Examples of such ontologies in the biomedical domain include the Foundational Model of Anatomy, SNOMED CT and the NCI Thesaurus. The goal of the Ontology Summit was not to establish a definitive definition of the word “ontology”, which has proven to be extremely challenging due to the diversity of artifacts it can refer to. Rather, the results of the Summit identified a limited number of key dimensions along which ontologies can