UN Security Council decisions impact billions of people and yet its formal rules are minimal and tell us little about how decisions are made. Instead, informal, and often unwritten practices, form the basis of negotiations. Inside the UN Security Council analyses informal practices within Security Council decision-making, both in general and focused on the case of Darfur in the west of Sudan, to pull back the curtain on decision-making. Security Council negotiations on Darfur are analyzed in depth across issue areas of agenda-setting, sanctions, referral to the International Criminal Court, and peacekeeping. One way of understanding these informal practices is via the lens of legitimation. This is a useful approach because it brings to the fore the ways in which states seek legitimacy for themselves, and for Security Council decisions, as part of the negotiation process. Inside the UN Security Council introduces and develops the concept of legitimation practices to analyse the UN Secur