This article is intended to clarify, for those who are not familiar yet with them, the commonalities and specificities of knowledge organization systems (KOS) on the one hand and ontologies on the other, considering their common structural model and their shared purpose as tools assisting in the organization and retrieval of information. In other words its aim is to define what is common and what separates knowledge organization systems from ontologies and why converting former KOSs into ontologies is preferred. In the end interoperability issues are discussed.