This chapter focuses on International Non-Governmental Organizations’ (INGO) legitimacy reflects not only increased demands for demonstrating outcomes and impact, but also for yielding greater control to supporters and constituents. INGOs face regular legitimacy challenges because many of them claim to stand for human rights, abide by principles such as impartiality, and pursue fundamental social or political change. The critiques directed at INGOs have led to greater self-reflection, but also to specific measures designed to enhance INGO legitimacy in the eyes of their stakeholders. INGO scholars draw on different disciplines to establish dimensions of legitimacy. The chapter explores how INGO legitimacy challenges are ultimately tied to questions about their basic purpose. It outlines the “4 Ps” model of legitimacy practices focused on purpose, people, practices, and performance. The model offers an attempt to conceptualize legitimacy in a way that enables empirical analysis across disciplines.