This chapter explains the research questions and point of departure for the book, provides a brief examination of the emergence and evolution of emissions trading, and presents the structure of the book. The book provides in-depth studies of how policy diffusion has influenced ETS design in most of the systems operational today. We pay particular attention to design convergence and divergence across systems and the relationship between diffusion mechanisms, domestic politics and such outcomes. This examination can help us understand the emergence and evolution of ETS around the world, how these systems spread, and ultimately the mechanisms and processes that influence their design and operation.