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is a simple tractable Description Logic that features conjunctions and existential restrictions. Due to its favorable computational properties and relevance to existing ontologies, has become the language of choice for terminological reasoning in biomedical applications, and has formed the basis of the OWL EL profile of the Web ontology language OWL. This paper describes ELK—a high performance reasoner for OWL EL ontologies—and details various aspects from theory to implementation that make ELK one of the most competitive reasoning systems for ontologies available today.