Traditionally, research on knowledge organization systems (KOS) and information retrieval discussed the relative advantages or disadvantages of using controlled vocabularies versus free-text or intellectual indexing versus automatic indexing methods for indexing and search. Experiments and case studies variously showed the superiority of either approach without reaching a final conclusion on this seemingly basic question. As full-text indexing has become more possible and now prevalent, the discussion of the relative merits of KOS – not only as substitute but in combination with full-text – was not settled but continued with new challenges. With the advent of the Semantic Web, KOS (now appearing as ontologies) became important tools in new information retrieval applications and were pushed once again to the research forefront. With different disciplines working in the field, the terminology around KOS has become more and more ambiguous up to the point that tracing research in the literature is difficult – ironically something that traditional KOS have always tried to mitigate. This paper summarizes recent discussions of the impact of KOS on information retrieval and attempts to show and unify different research strands from library science research on subject indexing, information retrieval and the Semantic Web. Whereas earlier impact studies on retrieval resulted in clearly measurable outcomes (for example changes in precision / recall), recent use of KOS in Semantic Web applications or other information systems has switched from pure search scenarios to exploration (browse) and contextualization, for which clear (and calculable) evaluation or quality standards and benchmarks do not exist.