It is argued that subsidiarity should be interpreted, in accordance with the principle of effectiveness, as requiring that the Community should only act where the objectives of the proposed action can only be achieved at Community level. Subsidiarity has not so far been an effective brake on action by the European institutions, and the Court’s scrutiny of Community acts for compliance with subsidiarity has been undemanding. The Constitution Treaty seeks to confirm and strengthen application of subsidiarity. Monitoring by national parliaments, and in particular the possibility for one third to object to a proposal on subsidiarity grounds, thus “showing a yellow card,” could lead to improved compliance with subsidiarity by the lawmaking institutions; and the “yellow card” procedure could change the dynamics of judicial enforcement of subsidiarity. Where national parliaments “raised a yellow card,” but the Commission maintained its draft, one possibility (which the present writer would advocate) would be that in any subsequent judicial proceedings the Court of Justice would require the Commission to demonstrate that the national parliaments had made a manifest error of appraisal in objecting to the draft act on subsidiarity grounds. Giving teeth to subsidiarity by entrusting national parliaments with responsibility for monitoring its application, and reinforcing that responsibility with an appropriate judicial response from the Court of Justice, could enhance the sense of “ownership” of the European project at national level. Although it appears unlikely that the Constitution Treaty will come into force, that fact need not prevent the introduction by other means of subsidiarity monitoring by national parliaments, and the adoption by the Court of Justice of the approach indicated.