The aim of the presentation is to illustrate how a deeper knowledge of students’ conceptions of a specific subject content can be explored. A phenomenographic study that explores qualitative differences in upper secondary school students’ conceptions of justice, in the subject of civics, is presented. The presentation discusses how the understanding of students’ different ways of reasoning about justice in civics can contribute to developing pedagogical content knowledge among teachers. The study is based on empirical data from seven interviews and three Learning Studies focusing the ability to reason about the concept of justice. The study analyses students’ conceptions of justice, as well as students’ experiences of what it means to reason in civics. These ways of reasoning relate to each other and to the practise of civics. By focusing critical aspects of the ability to reason about justice in communicative actions that enable the students to discern the critical aspects, the teachers develop a better understanding of what it means to be able to reason well about justice. The teachers also develop an understanding of how this can be taught to students.