For the second consecutive year, debate over carbon markets sullied the outcome of global climate change negotiations. COP25 in Madrid began with Article 6 on international mitigation cooperation as the only unfinished section of a rulebook for executing the Paris Agreement, and the wickedest problem on docket. A year of advance work and two weeks of deliberation proved insufficient, and Article 6 will again be front and center at COP26 in Glasgow in 2020. Why is Article 6 so contentious and what are the implications of another failure to resolve it?