International organizations (IOs) are important to handle major global challenges. However, the legitimacy of these organizations are often questioned and they need to convince stakeholders that they exercise authority in an appropriate way. Other state and non-state actors also engage in practices that may boost or challenge beliefs in the legitimacy of IOs. Problems of legitimacy and (de)legitimation are key to understanding contemporary global governance. Comprehensive studies of the linkages between different dimensions of legitimacy and (de)legitimation are missing. Given the rich quantitative and qualitative data I have collected on the legitimacy challenges of the Association of Southeast Asian Nations (ASEAN), I can make a unique contribution to research on legitimacy and (de)legitimation in global governance through a comprehensive analysis of relations between legitimacy beliefs and (de)legitimation discourses and actions. Demonstrating how the legitimacy challenges faced by ASEAN compares to those of other IOs will also enrich academic debates on ASEAN and international relations of the Asia-Pacific more broadly. Analysing existing data and engaging in additional data collection during stays in Indonesia, I will address the overarching research question: How and when is ASEAN legitimized and delegitimized and how does this relate to legitimacy beliefs about this organization? The project will result in a monograph and at least one journal article.