This paper analyses the themes of rationality and legitimation in the discourse of sustainable development. We start by constructing ideal types of interpretation of sustainable development (weak and strong) and rationality (value rationality and instrumental rationality) as conceptual and theoretical ‘tools’ for our further analysis. We then discuss the role of and analyze the construction of rationality and legitimation in the sustainable development discourse. We explore the ways in which rationality and legitimation are constructed to support one’s own interpretation, and, on the other hand, how the rationality and legitimacy of the opposing interpretations are nullified. We then discuss the basis of prudent decision-making, and the possibilities and problems that are bound up in the concept of sustainable development. The paper concludes by stating that when it comes to sustainable development neither societal decision-making nor the actions based on it is currently prudent.