This chapter shows that there is reasonable disagreement over some elements of distributive fairness in the UNFCCC. It specifically argues that there is reasonable disagreement over the fair distribution of emission rights in this context. It does this by analysing various principles of distributive fairness that have been advocated both in theory and in practice for what is fair in the UNFCCC, arguing that each of these principles represents a ‘reasonable’ proposal of what is fair. It does this by first developing a notion of ‘reasonableness’ based on liberal political theory and then comparing this standard against the most prominent principles that have been proposed in the UNFCCC. It then argues that, because the disagreement in the UNFCCC represents ‘reasonable disagreement’, it is very unlikely that parties will arrive at a mutually acceptable outcome through deliberative debate. This provides a platform for thinking about how to respond to reasonable disagreement, which is something I take up in Chap. 3.