Today civil society groups are important actors on the international stage. Nongovernmental organizations (NGOs) have taken roles that traditionally have been the sole province of states or intergovernmental institutions. NGOs are not bound to act in the public interest. Neither are their actions justified by formal democratic procedures, as is the case with states. Therefore, questioning the legitimacy of their actions is a crucial thing to do. This article presents the results of empirical research on the legitimacy of internationally operating (...) NGOs (INGOs). From the interview data seven types of legitimacy are distinguished. These do not give us a comprehensive categorisation of sources of legitimacy; rather they provide tools to counterbalance existing views of legitimacy. The aim is to develop concepts for evaluating the legitimacy of INGO activities which are grounded in theory as well as in practice. Before analysing the empirical results concerning NGO legitimacy, some views on civil society will be discussed with a focus on the problem of legitimacy. (shrink)