Abstract The article establishes three propositions. First, if a constitution establishes the principles of subsidiarity and proportionality as legal principles, questions of competencies are closely tied up with questions of regulatory policy. This means that the Treaty carves out a powerful role for the Court of Justice to assess the jurisdictional reasonableness of market intervention when reviewing whether the EU was legally competent to act. Second, general scepticism about courts being able to play such a demanding role in policing jurisdictional boundaries in federal systems are unjustified in the EU. The new procedure established in the Constitutional Treaty, which is likely to be included in any renegotiated constitutional settlement, involves national Parliaments and the Commission building a written record addressing the relevant policy issues on which the court can base its review. Additionally national courts serve as an external check on the Court of Justice, disciplining the Court of Justice to focus on taking competencies seriously or facing the prospect of national courts disapplying EU law on the grounds that it was enacted ultra vires. Third, even though there are some promising points of departure in its case law, the Court of Justice has not yet adopted a doctrinal framework that effectively operationalises the Treaty's commitment to subsidiarity and proportionality in the context of the common market.