Sustainability assessment (SA) is an increasingly popular term referring to a broad range of approaches to align decision-making with the principles of sustainability. Nevertheless, in public and private sectors sustainability results are still disappointing, and this paper reflects on this problem and proposes a way forward. We argue that, because sustainability issues are generally wicked problems (i.e. a ‘complex of interconnected factors in a pluralistic context’), effective assessments need to be reflexive about the definition of the issue and about the criteria for sustainable solutions. Based on a distinction of policy problems, we characterize SA as a form of problem structuring, and we distinguish three typical ways of problem structuring, corresponding to three different ways of integrating reflexivity in the assessment. We illustrate these routes in three examples. We discuss the way reflexivity is integrated in each example by discussing the mix of methods, SA process and epistemological balance. Rather than merely calling for more st akeholder participation, our aim is to call for more reflexivity integrated into the SA approach, and we conclude by proposing a process map for reflexive sustainability assessment to support this.