Treaty of Lisbon is signed by members states of European Union

The Treaty of Lisbon (initially known as the Reform Treaty) is an international agreement that amends the two treaties which form the constitutional basis of the European Union (EU). The Treaty of Lisbon was signed by the EU member states on 13 December 2007, and entered into force on 1 December 2009. It amends the Maastricht Treaty (1992), known in updated form as the Treaty on European Union (2007) or TEU, and the Treaty of Rome (1957), known in updated form as the Treaty on the Functioning of the European Union (2007) or TFEU. It also amends the attached treaty protocols as well as the Treaty establishing the European Atomic Energy Community (EURATOM).

Prominent changes included the move from unanimity to qualified majority voting in at least 45 policy areas in the Council of Ministers, a change in calculating such a majority to a new double majority, a more powerful European Parliament forming a bicameral legislature alongside the Council of Ministers under the ordinary legislative procedure, a consolidated legal personality for the EU and the creation of a long-term President of the European Council and a High Representative of the Union for Foreign Affairs and Security Policy. The Treaty also made the Union's bill of rights, the Charter of Fundamental Rights, legally binding. The Treaty for the first time gave member states the explicit legal right to leave the EU, and established a procedure by which to do so.

The stated aim of the treaty was to "complete the process started by the Treaty of Amsterdam [1997] and by the Treaty of Nice [2001] with a view to enhancing the efficiency and democratic legitimacy of the Union and to improving the coherence of its action". Opponents of the Treaty of Lisbon, such as former Danish Member of the European Parliament (MEP) Jens-Peter Bonde, argued that it would centralize the EU, and weaken democracy by "moving power away" from national electorates. Supporters argue that it brings more checks and balances into the EU system, with stronger powers for the European Parliament and a new role for national parliaments.