The Treaty of Lisbon is signed by the EU member states to amend both the Treaty of Rome and the Maastricht Treaty which together form the constitutional basis of the EU. The Treaty of Lisbon is effective from 1 December 2009.

The Treaty of Lisbon is a European agreement that amends the two treaties which form the constitutional basis of the European Union (EU). The Treaty of Lisbon, which was signed by all EU member states on 13 December 2007, 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, as well as 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).