This article traces some development lines from 20 years of the EU emission trading system (EU-ETS). Starting from the boundary conditions when the ETS Directive was adopted in 2003, it first traces the increasing shift of system accountability for the EU-ETS from the Member States to the EU level. Then, the development of two basic principles of EU emissions trading are considered: The importance of the ex-ante principle and the goals of free allocation of allowances. The story of the EU-ETS over the last 20 years may be told as the story of external shocks which were strong enough to create a common conviction for the need of structural changes within the EU-ETS.