Under pressure to abate greenhouse gas emissions without burdening their economies, several countries around the world have introduced emissions trading systems as a centerpiece of their climate change mitigation strategies. Drawing on the experiences with emissions trading made in Europe, North America, and the Asia-Pacific region, this article shows that considerable diversity can be observed across systems, providing valuable opportunities for comparison and policy learning. Individually, and in comparison, existing trading systems offer lessons that can be applied to the design and implementation of new systems – especially in emerging economies where carbon markets are currently under development, such as China – and to the improvement of already operating systems. Such lessons are identified in three different categories: the role of the political process and economic context; system design; and system implementation and operation.