Abstract This chapter gives an overview of the description logics underlying the OWL 2 Web Ontology Language and its three tractable profiles, OWL2 RL, OWL2EL and OWL 2QL. We consider the syntax and semantics of these description logics as well as main reasoning tasks and their computational complexity.We also discuss the semantical foundations for first-order and datalog rewritings of conjunctive queries over knowledge bases given in the OWL2 profiles, and outline the architecture of the ontology-based data access system Ontop.