The Treaty of Lisbon entered into force in the European Union.

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).

The European Union (EU) is a supranational political and economic union of 27 member states that are located primarily in Europe. The union has a total area of 4,233,255 km² (1,634,469 sq mi) and an estimated population of over 450 million as of 2025. The EU is often described as a sui generis political entity combining characteristics of both a federation and a confederation.

December 1 is the 335th day of the year in the Gregorian calendar; 30 days remain until the end of the year.