Background: Research on peer assessment has noted ambiguity among students in using peer assessment for improving their work. Previous research has explained this in terms of deficits in the student feedback, or differences in student views of what counts as high-quality work. Purpose: This study frames peer assessment as a social process in the science classroom. The aim is to explore peer assessment in science education as social practice in order to contribute to an understanding of the affordances and constraints of using peer assessment as a learning tool in science education. Design and Method: The study was conducted in four lower secondary school classes, school years 8 and 9, in two different schools. An intervention study was designed focussing on the topic of experimental design. It involved the students in a process of peer assessment where they designed experiments individually, and then exchanged their designs, conducted each other's experiments, provided feedback to each other and revised their original design after discussing the feedback in groups. Data were collected in the form of audio recordings of student discussions and written work. Results: The results show that, although not all peer feedback resulted in revisions, peer feedback was useful to the students in group interaction when negotiating quality in their work. Conclusions: To conclude, the potential for using peer assessment in science education should not only be evaluated through the students' revisions but also in terms of in what ways the feedback constitutes interactional resources for defining quality in student work.