A reasoner is a key component for working with OWL ontologies. In fact, virtually all querying of an OWL ontology (and its imports closure) should be done using a reasoner. This is because knowledge in an ontology might not be explicit and a reasoner is required to deduce implicit knowledge so that the correct query results are obtained. The OWL API includes various interfaces for accessing OWL reasoners. In order to access a reasoner via the API a reasoner implementation is needed. There following reasoners (in alphabetical order) provide implementations of the OWL API OWLReasoner interface:

• FaCT++.
• HermiT
• Pellet
• RacerPro (via OWLLink)

To use any of these reasoners you must download the appropriate libraries and place them in your class path. All of these reasoner implement the OWLReasoner interface and provide implmentations of the OWLReasonerFactory interface for instantiating the appropriate OWLReasoner implementation.