Background: Despite the renewed interest in citizenship education, relatively little is known about effective ways to realize citizenship education in the classroom. In the literature on citizenship education, dialogue is considered to be a crucial element. However, there is very little, if any, empirical research into the different ways to stimulate dialogue. Aim: The main aim of this study is to arrive at an understanding of how citizenship education can be integrated in history classes. The focus is on the effect of a dialogic approach to citizenship education on students' ability to justify an opinion on moral issues. Sample: Four hundred and eighty-two students in the eighth grade of secondary education. Methods: Two curriculum units for dialogic citizenship education were developed and implemented. The two curriculum units differed in the balance between group work and whole-class teaching. Students' ability to justify an opinion was assessed by means of short essays written by students on a moral issue. The effectiveness of both curriculum units was compared with regular history classes. Results: Students who participated in the lessons for dialogic citizenship education were able to justify their opinion better than students who participated in regular history lessons. The results further show a positive effect of the amount of group work involved. Conclusion: The results of this study indicate that a dialogic approach to citizenship education as an integral part of history classes helps students to form a more profound opinion about moral issues in the subject matter. In addition, group work seems to be a more effective method to implement dialogue in the classroom than whole-class teaching.