How do new forms of organizations legitimate themselves? Nongovernmental organizations (NGOs) have emerged as a dominant form of civil society organization in the past 35 years, often in societies undergoing social and political transitions. This paper analyzes legitimation processes for NGOs in postwar and postcommunist Croatia using interview and archival data. Starting from a principle that legitimation is a process involving multiple audiences, I examine the mechanisms NGOs use to legitimate themselves to different audiences. I find that Croatian civil society organizations adopted the NGO organizational form to legitimate themselves to international actors, who in turn pressured the Croatian government to accept these organizations as legitimate. However, the actions NGOs took to legitimate themselves to international actors undermined their legitimacy with the Croatian public. This case demonstrates that legitimation is a challenging prospect for a new type of organization, especially given the fragile nature of mediated legitimacy and the conflicting priorities of different audiences.