The principle of subsidiarity relates to legislative matters where the EU and member states have shared competence, and establishes the principle that action should be taken at the appropriate level (with the assumption that the EU only takes action if objectives can be more effectively achieved at EU rather than member state level). The EU Constitutional Treaty includes a Protocol which provides an 'early warning mechanism' under which national parliaments can challenge proposed legislation considered to breach the principle of subsidiarity. The Committee's report examines the principle of subsidiarity, the aims of the mechanism and attitudes of national parliaments to it, monitoring arrangements, and the role of the European Court of Justice (ECJ). Findings include that the principle is an important one which if applied correctly can act as a check on EU level actions whilst helping ensure effective action is taken where needed; and clarification is needed from the Government on the role of the ECJ.