An existing aid is defined as all aid which is (i) in force when a Member State joins the European Union, (ii) authorized by the Commission or by the Council, (iii) deemed to have been authorized or (iv) deemed to be an existing aid, as well as any aid which, when put into effect, did not constitute an aid but became and aid due to the evolution of the internal market (Article 1, Regulation No 2015/1589). The same goes for an aid which constitutes the strict and foreseeable application of the conditions laid down in a decision approving the general aid scheme or a notified new aid when the Commission has notified its decision late.

According to Article 108(1) TFEU, existing aid schemes are subject to continuous review by the Commission, which may propose to Member States appropriate measures required by the progressive development or functioning of the internal market. The review takes place in cooperation with the Member State which must provide the Commission with all necessary information for the review (Regulation No 2015/1589, Art. 21).

Where the Commission considers that an existing aid scheme is not, or is no longer, compatible with the internal market, it shall inform the Member State concerned of its preliminary view and give the Member State concerned the opportunity to submit its comments within a period of 1 month. Where, in the light of the information submitted by the Member State, the Commission concludes that the existing aid scheme is not, or is no longer, compatible with the internal market, it issues a recommendation proposing either a substantive amendment of the aid scheme, the introduction of a number of procedural requirements or the abolition of the aid scheme (Regulation No 2015/1589, Art. 22). Where the Member State concerned accepts the proposed measures, it is bound by its acceptance to implement them. If the appropriate measures are refused, the Commission opens a preliminary examination procedure for new aid (Art. 23). It may ultimately adopt an incompatibility decision, which will, however, only be valid for the future.