Not that long ago, hardly anyone would have thought that greenhouse gas emissions could, and would, be an object of trade and that such a process would help to reduce these emissions. Despite fierce criticisms directed at emissions trading, the system is in full operation in the context of climate change, and it is spreading around the globe. Emissions trading was first employed in the United States (US) and was then embraced by the United Nations Framework Convention on Climate Change (UNFCCC). Emissions trading systems were then created in a few countries in Europe before being adopted by the European Union (EU), and have recently also been introduced in Oceania and Asia. Seen from a proliferation perspective, emissions trading is a great success, having been created and crafted from nothing more than the virtual world of economic and legal ideas. Emissions trading systems can, of course, be used in other contexts to reduce other gases - such as sulphur dioxide (SO2) (which causes acid rain) or mono-nitrogen oxides (NOx) (which cause smog and acid rain) - but in this book we concentrate on climate change as its most prominent area of application at present. Gases responsible for climate change are carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), sulphur hexafluoride (SF6), and two groups of gases, hydrofluorocarbons (HFCs) and perfluorocarbons (PFCs).