Unreasonable expectations about the nature and character of scientific knowledge support the widespread political assumption that predictive scientific assessments are a necessary precursor to environmental decision making. All too often, the practical outcome of this assumption is that scientific uncertainty becomes a ready-made dodge for what is in reality just a difficult political decision. Interdisciplinary assessments necessary to address complex environmental policy issues invariably result in findings that are inherently contestable, especially when applied in the unrestrained realm of partisan politics. (...) In this article, the authors argue that predictive scientific assessments are inherently limited in the extent to which they can guide policy development and that rigorous scientific assessments can be much more valuable in the role of ex post policy evaluation than they can in the context of ex ante policy formulation. (shrink)