Socioscientific argumentation has shown to be a feasible educational framework for promoting citizenship and for cultivating scientific literacy. However, there are several aspects of this educational framework that have been shown to be problematic. Consequently, in this thesis I investigated various aspects of quality of socioscientific argumentation from both an upper secondary student and a teacher perspective. By using students’ written argumentation on socioscientific issues (SSI) I studied how they justified their claims. The results showed that different SSI led students to use different subject areas in their justifications. I also compared science majors with social science majors and found that the number of justifications provided by the students is related to their discipline background. In these two studies, a new content focused analytical framework for analyzing content aspects of socioscientific argumentation, the SEE-SEP model, was used and shown to be suitable for this purpose. However, to ensure that students are able to produce high-quality arguments I suggest that both content and structural aspects need to be considered. As a result of this, I have presented a framework based on research literature and the Swedish curriculum, for analyzing and assessing both these aspects of socioscientific argumentation. Moreover, I investigated how science and language teachers assess students’ socioscientific argumentation and found that the science teachers focused on students’ ability to reproduce content knowledge, whereas language teachers focused on students’ ability to use content knowledge from references, and the structural and linguistic aspects of argumentation.   The complexity of teaching socioscientific argumentation makes it difficult to teach and assess comprehensively. In order to promote quality and include both content and structural aspects, I suggest that a co-operation among teachers of different disciplines is beneficial. / Socioscientific argumentation has shown to be a feasible educational framework for promoting citizenship and scientific literacy. In this thesis I investigated various aspects of quality of students socioscientific argumentation and how teachers assess this. The results showed that different SSI led students to use different subject areas in their justifications and that the number of justifications provided by the students is related to their discipline background. Moreover, to promote students high-quality arguments I have presented a framework for analyzing and assessing both content and structural aspects. I also investigated how science and language teachers assess students’ socioscientific argumentation and found that the science teachers focused on students’ ability to reproduce content knowledge, whereas language teachers focused on students’ ability to use content knowledge from references, and the structural and linguistic aspects of argumentation. The complexity of teaching socioscientific argumentation makes it difficult to teach and assess comprehensively. In order to promote quality and include both content and structural aspects, I suggest that a co-operation among teachers of different disciplines is beneficial. / <p>Article IV was in manuscript form at the time of the thesis defense and has been published afterwards.</p>