This chapter examines the proliferation of emissions trading, which has increasingly been adopted as a response to the 'super-wicked' climate problem. It presents in-depth studies of how policy diffusion has influenced emissions trading systems (ETS) design in most of the systems operational today. The chapter also examines diffusion as a causal mechanism and the link between diffusion mechanisms and design outcomes. It investigates the international frontrunner European ETS, which is so far the largest carbon market in the world and an important provider of experience in how to design a carbon market. The chapter focuses on the first subnational carbon market in the United States, the Regional Greenhouse Gas Initiative on the East Coast. It analyses the emergence and evolution of emissions trading in China – the biggest greenhouse gas emitter globally. The chapter also presents some closing thoughts on the key concepts discussed in the subsequent chapters of the book.