Environmental policymaking has been equated with the art of making the right decisions based on an insufficient understanding of the underlying problems. Given the tremendous complexities of the earth system, effective global environmental governance must rely on scientific information on both the kind of problem at stake and the options for decisionmakers to cope with it. Some global environmental problems, notably stratospheric ozone depletion, climate change, and biodiversity conservation, have thus sparked off formidable increases in scientific research. Remarkable too is the enormous expansion of international scientific cooperation for assessing this accumulated knowledge and for synthesizing it in a form accessible and useful to decisionmakers. About 2,500 scientists, for example, have been working with the United Nations-sponsored Intergovernmental Panel on Climate Change for its three-volume assessment report Climate Change 1995, (1) and the Global Biodiversity Assessment of 1995 (2) involved roughly 1,500 experts in this field. The immense networks of scientists, experts, national governments, private bodies, and international organizations engaged in these major global environmental assessments can be understood as distinct international institutions within the larger endeavor of global environmental governance, consisting of internationally accepted general principles for producing, synthesizing, and legitimizing expert knowledge; international norms and rules regulating this synthesis and the evaluation of knowledge in specific cases; and pertinent decisionmaking procedures. The main function of these institutions is not environmental protection as such, but comprehensive and reliable scientific advice on the state of the environment and on policy options, which reduces transaction costs for governments. (3) In a world society that is becoming increasingly interlinked and interdependent, science and expert advice is needed to inform decisionmakers on the complex problems they face, leading to a likely increase in the relevance of scientific advisory institutions in international relations. But although enormous efforts are undertaken in these scientific advisory institutions, political scientists need to question their actual effects both on national decisionmaking and on international politics. It is not yet fully understood what exact role the existing advisory institutions play in the course of global environmental governance and how this influences international politics and national decisionmaking. The debate on regimes and institutions in global governance has long concentrated on the general effects of institutions on national decisionmaking, (4) but only recently has the specific impact of scientific advisory institutions received attention from students of international relations. In the early 1990s, for example, it was shown that an epistemic consensus about the interpretation of science reached within international assessment processes influences negotiations and may help create international environmental regimes. (5) More generally, political scientists have worked on the role of ideas in international relations, (6) on information systems within environmental regimes, (7) and on the social construction of science for policy in global environmental regimes. (8) Likewise, substantial research has been directed to the impact of international scientific advisory institutions on the political process in industrialized countries in the areas of climate change and regional air pollution. Most of this research has indicated a generally significant impact of advisory institutions on the behavior of actors in the North. (9) Another interesting finding of this research is that noticeable differences in effectiveness exist between Western industrialized countries and the countries in transition to a market economy in Eastern Europe. …