Decentralization of the energy sector means the breaking-up of the sector and its vertically integrated enterprises and/or global cartels by separating its distinct functions (extraction, transmission and sale), thereby allowing for increased competition in the market. This chapter uses two case studies to illustrate the challenges the decentralization of largely vertically integrated energy markets poses for international trade law, including international trade law’s inability to deal comprehensively with the production quota practices of global energy cartels such as the Organization of Petroleum Exporting Countries (OPEC). The chapter then studies regional energy market decentralization policies (in this case the European internal market), focusing on the panel report in EU – Energy Package, and considers which WTO rules facilitate such policies and which constrain them.