International institutions are facing a new paradox: there is an urgent need for them, but they are increasingly being resisted, and one of the main grounds for supporting and resisting international institutions is their legitimacy. Every organization claims legitimacy for itself and denies the legitimacy of others. However, a framework for evaluating legitimacy is lacking. This paper defines the concept of legitimacy for international institutions and suggests that it derives from four sources (normative values, comparative benefits, national recognition, and confirmation by other international institutions). Based on their relevancy and their capacity for operationalization in this evaluation, indicators for input, operation, and output legitimacy have been selected to methodically evaluate the legitimacy of international institutions.