The motivation for writing this book was when working with large ontologies, the problem was encountered that common reasoners are not able to execute reasoning tasks anymore. They soon reach their limit by running out of memory or already fail on loading huge data. Thus the idea of performing reasoning with (large) ontologies in databases arose. In this book, a short introduction to Ontologies and Description Logics is provided. The main part includes following topics: firstly, the description of the process of mapping ontologies into relational databases is given. Subsequently, common reasoning tasks (e.g. consistency / instance checks, ontology classification) are replicated for usage in the database. The implementation of those tasks is realized in SQL (and PL/SQL) and illustrated with examples. In order to reach even better performance in the database, some optimization strategies are presented, for example the effective usage of index structures. Finally, some evaluation was done by executing performance tests with different example ontologies, resulting in a comparison of existing reasoners (e.g. Pellet) and the database performance.