The recognition that information matters in world affairs raises a number of questions as to when, how, and under what conditions it influences the behavior of policy actors. Despite the vast and growing array of institutions involved in collecting, analyzing, and disseminating information potentially relevant to global governance generally, and global environmental change specifically, our understanding of the role that these “information institutions” play in world affairs remains limited. This paper examines how institutions mediate the impact of scientific assessments on global environmental affairs and highlights the pathways through which information has influence on the policy and politics of environmental issues. We identify salience, credibility and legitimacy as the critical attributions that different audiences make about an assessment that determine whether they will change their thoughts, decisions, and behavior in response to it. We also outline how institutional rules regarding participation, framing, and scope and content allow knowledge systems to reach needed thresholds of salience, credibility, and legitimacy and to balance the tradeoffs and tensions among them.