The EU emissions trading system (ETS) is the first large-scale international emissions trading system and a “cornerstone” in EU climate policy. A key element in the ETS implementation process is deciding upon the ceiling (“cap”) for the emissions included in the ETS. Over time, a significant change and centralization of this model has taken place. In order to understand this development, we need to acknowledge the increasing acceptance of stronger centralized governance among the member states due to ETS pilot phase problems; take into consideration frustration in the European Commission over complex and differing National Allocation Plans; and add the fact that the Kyoto Protocol target was getting nearer and a good performance of the “flagship” ETS was becoming increasingly important. Hence, although the case supports the importance of acknowledging the multilevel character of the EU, it still emphasizes the key role of changes in member states' interests and positions for understanding outcomes.