In an "age of measurement" where students' "qualification" is a hot topic on the political agenda, it is of interest to ask what the "function of qualification" might implicate in relation to a complex issue as Education for Sustainable Development (ESD) and what function environmental and sustainability issues serve in science education. This paper deals with how secondary and upper secondary teachers in discussions with colleagues articulate qualification in relation to educational aims of ESD. With inspiration from discourse theory, the teachers' articulations of qualification are analysed and put in relation to other functions of education (qualification, socialisation and subjectification). The results of this study show three discourses of qualification: "scientific reasoning," "awareness of complexity" and "to be critical." The discourse of "qualification as to be critical" is articulated as a composite of differing epistemological views. In this discourse, the teachers undulate between rationalistic epistemological views and postmodern views, in a pragmatic way, to articulate a discourse of critical thinking which serves as a reflecting tool to bring about different ways of valuing issues of sustainability, which reformulates "matter of facts" towards "matter of concerns."