In December 2008, the EU Emissions Trading System (EU ETS) was signifi cantly revised and strengthened. This article explores the basis for, and the consequences of, the revision for legitimacy. The key to legitimate EU governance is seen in the convergence of different sources of legitimacy at various levels of society. In addition to member-state consent, participation of non-state actors, democracy, expertise and effectiveness are of relevance.The first conclusion is that the recent revision of the EU ETS has indeed been grounded in a broader multilevel legitimacy basis. Second, the system faces signifi cant challenges with regard to carbon markets and effectiveness, which could reduce its legitimacy in the long term.