Journal of Environmental Policy & Planning Vol. 5, No. 3, September 2003, 285–309 Sustainable Development: Impact Assessment in the Age of Networking SIBOUT NOOTEBOOM & GEERT TEISMAN ABSTRACT 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 incorpor- ate 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. Introduction: Learning to Deal with Complexity Impact Assessment is often proposed with the aim of making decisions that contribute to sustainable development, as will be shown in this article. However, its practitioners meet a number of fundamental problems that are related to complexity. In this section some of these problems are introduced. The Fuzziness of Collective Interventions for Sustainable Development It is widely assumed that societal developments can lead to a tragedy of the commons. Present societal and governmental actions do not always help to strike a balance between the three objectives of sustainable development: economic growth, social cohesion and ecological improvement (e.g. Brundlandt, 1987). If neither market forces nor specific governmental organisations (responsible for Corresponding author: Sibout Nooteboom, PO Box 1076, 3833 KH Amersfoort, The Netherlands. Email: sibout. nooteboom@dhv.nl 1464-9357 Print/1470-000X Online/03/030285-25  2003 Taylor & Francis Ltd DOI: 10.1080/1523908032000154205