This thesis explores Civics teachers' assessment practices regarding aims, content, skills and methods, and factors that may affect teachers' assessment practices. This was examined through thematic analyses of interviews with a total of 19 upper secondary school teachers and of their assignments. The studies showed a variation, both between the teachers, and that individual Civics teachers varied their assessments. The teachers in the study had varied purposes, but a performative discourse, emphasising the grading criteria, resulted in a summative practice focusing on evidence for grading. On the other hand, the teachers expressed a high degree of agency regarding content, skills and methods. However, a subject tradition affected what content is tested. Historically, teachers have mainly tested factual knowledge in written tests, but the teachers in these studies varied their methods and emphasized reasoning skills, which is in line with the open character of the subject.The focus on grading criteria is positive for the validity of grading, but validity in relation to formative purposes can be problematized. However, the emphasis on summative assessment entails a risk of an instrumental view of school, focusing on grades rather than learning. Variation between teachers also risks problems arising regarding equal assessment, where students can learn different things and develop different skills. On the other hand, variation may help more students to succeed, as different methods suit different students.The teachers were socialised into assessment practices, which have been shaped primarily by their experiences, rather than teacher education. However, education in assessment could increase teachers' assessment competence. Assessment is difficult and complex, and teachers need to balance different purposes, choose appropriate methods in relation to those purposes and what content and skills are to be tested, as well as deal with the various consequences of assessments. It is important work, in which Civics teachers may achieve a high degree of agency and professionalism.