The 24th international climate conference in Katowice, Poland, in December 2018 was a major achievement in the multilateral response to climate change. More than 190 countries managed to agree on nearly all elements of a comprehensive rulebook that puts flesh on the bones of the 2015 Paris Agreement. The rules require, for the first time, that all countries provide detailed information on their climate change mitigation targets and regularly report on their progress in implementing and achieving them. However, one important chapter is still missing: rules for international carbon markets discussed under Article 6 of the Paris Agreement. Competing views on how to avoid “double counting”—counting the same emission reduction more than once to achieve climate mitigation targets—were a major roadblock to reaching consensus. Completing the missing chapter on Article 6 will be one of the key tasks when countries reconvene at the 25th international climate conference in Santiago, Chile, in December of this year. We highlight why resolving double counting is critical for achieving the goals of the Paris Agreement and identify essential ingredients for a robust outcome that ensures environmental effectiveness and facilitates cost-effective mitigation.