This article contributes to bridging the gap between theories on Impact Assessment and theories on (complex) decision making. Impact Assessment theories assume that causal impact analysis may provide information that contributes to better decision making in complex situations. These are situations where alternative courses of action have complex ramifications for impact. However, so-called rational knowledge is often available through Impact Assessment, but not used in decision making. Explanations can be found in two kinds of logic characterizing collective decision making. First, societal problems are complex and, therefore, multi-rational. Providing more information does not help to reduce complexity or to overcome multi-rationality. Second, the increasing need to co-operate results in high process dynamics, where irrationality and emotions prevail and information is gathered to underpin these 'irrationalities'. At present, Impact Assessment seems to be insufficient to attain the societal transitions that may be necessary for genuine sustainable development. The authors argue that Impact Assessment should incorporate tenable assertions about the logic of the decision-making processes in which the assessments should be used. Only, perhaps, in such circumstances can a more effective process based on multi-rational argumentation and plausibility be achieved. It is the authors' conviction that an improved understanding of multi-rational processes can contribute significantly to new types of Impact Assessment that more effectively support sustainable development.