The principle of subsidiarity is defined in Article 5 of the Treaty on European Union. It ensures that decisions are taken as closely as possible to the citizen and that constant checks are made to verify that action at Union level is justified in light of the possibilities available at national, regional or local level. Specifically, it is the principle whereby the Union does not take action (except in the areas that fall within its exclusive competence), unless it is more effective than action taken at national, regional or local level. It is closely bound up with the principle of proportionality, which requires that any action by the Union should not go beyond what is necessary to achieve the objectives of the Treaties.
The Edinburgh European Council of December 1992 issued a declaration on the principle of subsidiarity that laid down the rules for its application. The Treaty of Amsterdam took up the approach that followed from this declaration in a Protocol on the application of the principles of subsidiarity and proportionality. Following the entry into force of the Treaty of Lisbon on 1 December 2009, the Protocol now requires the principle of subsidiarity to be respected in all draft legislative acts and allows national parliaments to object to a proposal on the grounds that it breaches the principle, as a result of which the proposal may be maintained, amended or withdrawn by the Commission, or blocked by the European Parliament or the Council. In the case of a breach of the principle of subsidiarity, the Committee of the Regions may also refer directly to the Court of Justice of the European Union.