This thesis deals with the concept of accountability in international nongovernmental organizations INGOs). The four chapters focus on the theoretical and practical implications of a comprehensive approach to INGO accountability. Comprehensive INGO accountability goes beyond the donor-centric focus on financial accountability, and further includes outcome assessment processes and stakeholder dialog. The findings suggest that INGOs that implement such processes yield more effective programs as well as a stronger mission orientation. This implies that accountability is not merely to be understood as a necessary evil that occurs separately from the organization's core operations. Instead, comprehensive INGO accountability is integrated in organizational processes, and has the strategic value of strengthening organizational performance and mission orientation. It is based on dialog, and therefore is constructive in nature and contributes to more democratic organizational decision-making. In light of the increasingly political role of INGOs, implementing comprehensive accountability can be expected to become indispensable to ensure their organizational legitimacy.