My main research interests are knowledge representation and automated reasoning, and specifically, reasoning support for description logics, ontology languages, such as OWL, and decidable fragments of first-order logic. I have been actively working on the topic of modularity in ontologies together with Bernardo Cuenca-Grau, Ian Horrocks, and Uli Sattler, and also did some theoretical research on extensions of description logics, modal logics, and ontology languages with expressive features, such as complex role inclusion axioms, graded modalities / counting, role conjunctions, and conjunctive queries. Studying theoretical properties of various reasoning tasks is an exciting research area, but it is even more satisfying to develop procedures that work in practice. I have been involved in the development of a few ontology reasoning systems, such as CB, ConDOR, and ELK, which implement optimized consequence-based reasoning procedures, and I am interested in almost all aspects of algorithm optimizations, such as efficient data structures, goal-directed, incremental reasoning, and concurrency.