Scientific policy advice is usually perceived as a formalized advisory process within political institutions. Politics has benefited from this arrangement because the science-based rationalization of policy has contributed to its legitimacy. However, in Western democratic societies, scientific expertise that is routinely mobilized to legitimate political positions has increasingly lost its power due to controversial expertise in the public sphere in particular within the mass media. As a consequence of the medialization of science, political decision makers are increasingly confronted with mass-mediated (...) expertise. Empirical results of a qualitative survey among decision makers in the German political and administrative system, presented in this article, support the hypothesis that mass-mediated expertise has a significant impact on policy processes. Five functions of media coverage on science-based issues for policy making were identified. Mass-mediated expertise has therefore altered the established relations between scientific policy advisors and political decision makers and can be seen as informal policy advice complementing institutionalized advisory arrangements. (shrink)