During the last few decades, the discourse of sustainable development has motivated new political strategies for action in the environmental arena. One such strategy, used in several countries, is management-by-objectives, which means that objectives are formulated, their attainment is directed and the results are measured. In Sweden, which is frequently cited as a world-leading country as regards environmental policy, efforts to achieve sustainability have been canalised through sixteen national environmental quality objectives. The process of monitoring progress towards the environmental objectives relies on scientific theories and methods. Nevertheless, choices of e.g. indicators to measure goal achievement and methods for data analysis are by no means uncomplicated. Different ways of conceiving of -good science-, -usefulness- and -expertise- are competing in the system that has been developed to attain the environmental objectives. In this paper, I present results from a Swedish case study which encompasses in-depth interviews and focus group discussions with scientists and civil servants involved in the process of assessing progress towards the environmental objectives. I will address questions such as: What types of knowledge are judged to be relevant, credible and legitimate in the assessment of environmental goal achievement? How do scientists, practitioners and policy makers position themselves and others? How is scientific knowledge communicated between different actors? How can supply of and demand for knowledge be reconciled in the assessment process?