Pursuant to the precepts of EU law, EU policy-makers are bound to ensure that any EU legislation must fall within the remit of the EU's competences. This monograph looks at this highly contested issue, with particular reference to European Union criminal law. It looks at the powers enjoyed by the EU to impose criminal sanctions to suggest mechanisms by which legislative powers could be kept in check. The book argues that the main responsibility for providing checks against the exercise of EU power lies with the EU judiciary. It argues that the most effective form of review is procedural and through the case study of sanctions, provides the basis for such a review. Innovative, engaging and rigorous, this is an important publication both in the field of European criminal and constitutional law.