Sustainability science poses severe challenges to classical disciplinary science. To bring the perspectives of diverse disciplines together in a meaningful way, we describe a novel methodology for sustainability assessment of a particular social-ecological system, or country. Starting point is that a sustainability assessment should investigate the ability to continue and develop a desirable way of living vis-à-vis later generations and life elsewhere on the planet. Evidently, people hold different values and beliefs about the way societies sustain quality of life for their members. The first step, therefore, is to analyze people's value orientations and the way in which they interpret sustainability problems i.e. their beliefs. The next step is to translate the resulting worldviews into model-based narratives, i.e. scenarios. The qualitative and quantitative outcomes are then investigated in terms of associated risks and opportunities and robustness of policy options. The Netherlands Environmental Assessment Agency (PBL) has followed this methodology, using extensive surveys among the Dutch population. In its First Sustainability Outlook (2004), the resulting archetypical worldviews became the basis for four different scenarios for policy analysis, with emphases on the domains of transport, energy and food. The goal of the agency's Sustainability Outlooks is to show that choices are inevitable in policy making for sustainable development, to indicate which positive and negative impacts one can expect of these choices (trade-offs), and to identify options that may be robust under several worldviews. The conceptualization proposed here is both clear and applicable in practical sustainability assessments for policy making.