In the European Union (EU), enhanced cooperation (previously known as closer cooperation) is a procedure where a minimum of nine EU member states are allowed to establish advanced integration or cooperation in an area within EU structures but without the other member states being involved. As of October 2017, this procedure is being used in the fields of the Schengen acquis, divorce law, patents, property regimes of international couples, and European Public Prosecutor and is approved for the field of a financial transaction tax.

This is distinct from the EU opt-out, that is a form of cooperation between EU member states within EU structures, where it is allowed for a limited number of states to refrain from participation (e.g. EMU, Schengen Area). It is further distinct from Mechanism for Cooperation and Verification and permanent acquis suspensions, whose lifting is conditional on meeting certain benchmarks by the affected member states.

Enhanced cooperation, at that time known as closer cooperation, was introduced by the Treaty of Amsterdam for community, judicial cooperation and criminal matters. The Treaty of Nice simplified the mechanism: the right of veto which the Member States enjoyed over the establishment of enhanced cooperation has disappeared (except in the field of foreign policy), the number of Member States required for launching the procedure has changed from the majority to the fixed number of eight Member States. It also introduced cooperation for the Common Foreign and Security Policy, except for defence matters. At the same time, it stipulated that acts adopted within an enhanced cooperation do not form part of the Union acquis that new member states have to adopt. It also renamed closer cooperation to enhanced cooperation. The Treaty of Lisbon extended cooperation to include defence and additionally envisions the possibility for establishment of a permanent structured cooperation in defence. A minimum requirement of nine member states was also introduced. The provisions governing enhanced cooperation are now detailed in the Treaty on European Union (Article 20) and Treaty on the Functioning of the European Union (Article 326-334).

The Schengen Agreement adoption is considered a historical inspiration for formalising the mechanism of Enhanced cooperation. It was created by European Communities member states only, but outside of its structures, in part owing to the lack of consensus amongst all member states over whether it had the competence to abolish border controls, and in part because those ready to implement the idea did not wish to wait for others. As there was no Enhanced cooperation mechanism back then it was impossible to establish it inside the Community structures from the start, but afterwards the Schengen Agreement was subsumed into European Union law by the Treaty of Amsterdam as the rules of the Schengen Area.

Enhanced cooperation allows for a minimum of nine member states (which amounts to one-third at the moment) to co-operate within the structures of the EU without all member states. This allows them to move at different speeds, and towards different goals, than those outside the enhanced cooperation area. It is designed to overcome paralysis, where a proposal is blocked by the veto of an individual state or a small group who do not wish to be part of the initiative. It does not however allow for an extension of powers outside those permitted by the treaties of the European Union and is only allowed as a last resort where objectives cannot be achieved normally. It may not discriminate against member states, it must further the objectives in the treaties and may not fall within an area which is of exclusive competence of the EU.

The mechanism needs a minimum of nine Member States, who file a request with the European Commission. If the Commission accepts it then it has to be approved by a qualified majority of all member states to proceed. A member may not veto the establishment of enhanced cooperation except for foreign policy.

The Schengen acquis was originally established on an intergovernmental basis, but was later on integrated as an enhanced cooperation (at that time known as closer cooperation) into the framework of the European Union by the Treaty of Amsterdam. Ireland and the United Kingdom obtained opt-outs from the Schengen cooperation, allowing them to opt out from legal acts that build upon the Schengen acquis on a case-by-case basis. Formally speaking, the other member states are authorized to establish an enhanced cooperation among themselves and make use of the treaty provisions on such cooperations whenever Ireland (and previously also the United Kingdom) choose to opt out from a legal act that build upon the Schengen acquis.

However, the provisions on enhanced cooperation for the Schengen acquis differ somewhat from the provisions for other enhanced cooperations. For instance, the member states are automatically authorized to initiate an enhanced cooperation as soon as Ireland opt out; no separate decision has to be made in the Council. Furthermore, the Schengen acquis forms an integral part of the Union law that every acceding member state has to adopt, reflecting the provisions on enhanced cooperations in force before the Treaty of Nice.