The idea of fossil fuel subsidy reform (FFSR) can be considered as an international norm. Norms define what actors ought and ought not to do – respect human rights, for example, or ban chemical weapons. Contrary to binding laws and rules, norms are obeyed not (necessarily) because they are enforced, but because they are seen as legitimate. This chapter seeks to explain the emergence and (uneven) diffusion of FFSR as an international norm. Our focus lies on the international level, rather than on the domestic level. We first trace the long history of multilateral efforts to address fossil fuel subsidies, including at the G20 and the United Nations (UN). Then we interpret the role of norm entrepreneurs, political opportunity structures, and discursive contestation as drivers of the dynamics of the FFSR norm. A key conclusion that emerges from this is that the norm of FFSR remains essentially contested. In contrast to the established international consensus over how to define agriculture and fisheries subsidies, no common definition of energy subsidies has emerged, which hinders the implementation of the norm.