The aim of this article is to assess and explain the effectiveness of the international climate regime in a problem-solving perspective, with a focus on mitigation. As CO2 emissions have increased by more than 60 per cent since the start of the climate negotiations, effectiveness is exceedingly low. In explaining the performance of the regime, the main focus is on its problem-solving ability, defined as a function of power, leadership, and institutional design. ‘Negative’ power and a lack of leadership constitute important reasons for low effectiveness. In this broader perspective, the role of institutional design, exemplified by the Paris Agreement and its Rulebook, is fairly modest, and its significance should not be exaggerated. The Agreement and Rulebook score high in terms of ambition, but whether the rules will ever realize those ambitions remains to be seen. Domestic interests and priorities of the most important emitting countries will be decisive in this regard.