Major CO2 emitters such as individual states of the USA, the People’s Republic of China and the European Union continue to rely on emissions trading systems to reduce CO2 emissions. Against this background, this thesis examines four emissions trading systems in the USA. For this purpose, the legal and economic fundamentals of emissions trading are presented in detail. Subsequently, emissions trading systems are analyzed from an economic as well as a legal point of view based on criteria defined for this purpose and the results are embedded in the context of national and international law. For this purpose, the author conducts extensive basic research by processing the available emissions and trade data.