Ontologies are already used in the life sciences and the Semantic Web, but are expected to be deployed in many other areas in the near future-for example, in software development. As the use of ontologies becomes commonplace, they will be constructed more frequently and also become more complex. To cope with this issue, modularization paradigms and reuse techniques must be defined for ontologies and supported by ontology languages. In this paper, we propose the use of roles from conceptual modeling for this purpose, and show how they can be used to define ontological reuse units and enable modularization. We present role-based ontologies as an extension of standard ontologies and define their semantics through a reduction to standard Description Logics, such that existing reasoners can be used.