Despite the substantial reform of the EU ETS for the 2013–2020 phase adopted back in 2008, subsequent problems of allowance surplus and a low carbon price have spurred new efforts to further tighten the system. But these efforts have met resistance both among member states and in the European Parliament, and the EU has been struggling in its efforts to improve the ETS. This chapter draws on four central EU and political science theory approaches to more systematically explore why. The financial crisis and slow international policy progress have narrowed the window of opportunity that was open in 2008. An economic upswing, a new Commission and Parliament, and new global negotiations in 2015 are among the factors that could open that window again. But even without short-term reform the linear reduction factor will gradually tighten the system and lead to a higher carbon price.