The European Union emission trading system has been criticized by various stakeholders. We review the main criticisms and end up with the following conclusions: abatement has been real, albeit small, no noticeable loss of competitiveness or carbon leakage occurred, the distributional effects were unfair and major fraud occurred. Some of these problems could have been avoided and can still be corrected by adopting a more critical approach toward flexibility mechanisms and setting a CO2 floor price.