Concept representation is still an open problem in the field of ontology engineering and, more in general, of knowledge representation. In particular, it still remains unsolved the problem of representing "non classical" concepts, i.e. concepts that cannot be defined in terms of necessary and sufficient conditions. In this paper we review empirical evidence from cognitive psychology, which suggests that concept representation is not an unitary phenomenon. In particular, it seems that human beings employ both prototype and exemplar based representations in order to represent non classical concepts. We suggest that a similar, hybrid prototype-exemplar based approach could be useful also in the field of formal ontology technology.