This article serves as an introduction to a special section on the question of the legitimacy of non-state armed groups. Starting with a short discussion of the literature on armed groups as political actors, the authors emphasize the importance of the often-underestimated dimension of legitimacy. After having conceptualized legitimacy in more detail, the article addresses three key chal-lenges armed groups usually face regarding the politics of legitimacy: first, they need to legitimize the use of violent means; second, for moral and ma-terial support, they depend on beliefs of legitimacy; and third, they need to simultaneously address various domestic and international audiences. Finally, the authors highlight a number of pending questions for further research on armed groups.