The emissions trading scheme (EU ETS) adopted by the European Union in 2003 was a new instrument for the EU and its Member States. It is one of the most important strategies of achieving the EU's greenhouse gas reduction target under the Kyoto Protocol. This book analyses the policy cycle of the EU ETS Directive, focusing on the crucial implementation phase. The revised EU ETS Directive of 2009 includes significant changes for greater ecological effectiveness, changes that were unlikely to have been adopted in 2003. It is evident that the experiences of the first phase influenced not only the second implementation phase but also the revision. The intensive learning process that took place on all levels was necessary to overcome institutional constraints so that the EU ETS could be successfully established and further developed.The EU ETS policy-making is a good example to demonstrate that output legitimacy challenges input legitimacy: With the centralisation of the EU ETS in 2013, it is likely to become a more effective system; however, the legislators from the Member States may lose influence. This problem will have to be addressed.