Science for sustainability policy requires the handling of multi-dimensional and multi-scale analyses. Integrated assessment is about generating information relevant for decision-making. This generates a divide between two scientific paradigms: (1) 'Post-Normal Science' acknowledges the unavoidable existence of non-equivalent perceptions and representations of the reality; legitimate but contrasting perspectives found among social actors; heavy levels of uncertainty. (2) 'Normal Science', believes that it is possible to handle in a rigorous and rational way these challenges and that therefore it is possible to define in substantive terms 'the best course of action' for society. This paper is written to explain the reasons and the tools developed by scientists working within the Post-Normal Science paradigm. © 2005 Elsevier Ltd. All rights reserved.