Abstract. Despite of their advertisement as task independent representations, the reuse of ontologies in different contexts is difficult. An explanation for this is that when developing an ontology, a choice is made with respect to what aspects of the world are relevant. In this paper we deal with the problem of reusing ontologies in a context where only parts of the originally encoded aspects are relevant. We propose the notion of a viewpoint on an ontology in terms of a subset of the complete representation vocabulary that is relevant in a certain context. We present an approach of implementing different viewpoints in terms of an approximate subsumption operator that only cares about a subset of the vocabulary. We discuss the formal properties of subsumption with respect to a subset of the vocabulary and show how these properties can be used to efficiently compute different viewpoints on the basis of maximal sub-vocabularies that support subsumption between concept pairs.