The study explores the meaning of knowing how to reason about justice in civics in upper secondary school. This is examined through the analysis of students’ conceptions of justice, and of their conceptions of civic reasoning about justice. It is also examined through the analysis of civic education practise. In the study, teaching was designed using Variation Theory and the theory of intersubjectivity in Activity Theory, and examined and evaluated in three Learning Studies. The empirical material consists of filmed and transcribed research lessons and interviews, as well as of written pre- and post-tests. The material was first analysed using phenomenography, and then analysed using content-oriented conversation analysis. Students’ conceptions of justice were found to move between the conception of justice as (A) a universal value, (B) a personal value or (C) a value of principle. Students’ conceptions of civic reasoning about justice were found to move between three conceptions: (a) reporting about justice, (b) analysing causes of different perspectives on justice or (c) critical reasoning about principles of justice. The critical aspects of knowing how to reason about justice in civics that students needed to discern were the relativity of justice, the basis for arguments for principles of justice and the analysing as well as the criticizing aspects of reasoning. The subject-knowledge that the teachers expressed in their teaching was one condition of the practise of civics that was found to be of importance for the students’ learning. Others were the assumed purpose of the practise of civics and a genuine need for the intended knowledge in the practice. Communicative actions that seem to facilitate these conditions are real learning tasks and a subject-specific language and variation of critical aspects as mediating tools. The findings are discussed in relation to theories of justice in political science and to the practise of civics education.