On 13 December 2007, the heads of state and government of the European Union signed the Treaty of Lisbon (formally the Treaty on European Union and the Treaty establishing the European Community as amended) in Lisbon, Portugal. The treaty was the product of years of negotiation following the failure of the European Constitution in 2005 and aimed to update the EU’s legal framework to reflect its expansion to 27 member states. Key institutional reforms included the creation of a long-term President of the European Council, a new High Representative for Foreign Affairs and Security Policy who would combine roles previously split between the Council presidency and the European Commission, and changes to the composition and voting procedures in the Council of Ministers. Qualified majority voting was extended in many policy areas, while the treaty also reallocated certain competences between the EU and member states. The treaty amended the EU’s two founding treaties rather than replacing them with a single constitutional document, a choice intended to ease ratification in member states wary of overt constitutional symbolism. The Lisbon Treaty also increased the powers of the European Parliament through a broader use of the ordinary legislative procedure (formerly co-decision), enhancing the role of directly elected representatives in shaping EU law. It clarified citizens’ rights by introducing the Charter of Fundamental Rights into the treaty’s legal framework — with caveats for a few member states — and provided for a formal citizen-initiative mechanism allowing one million EU citizens from a significant number of member states to request the Commission to propose legislation. Ratification proved uneven and contentious. Ireland held two referendums: an initial rejection in 2008 was followed by approval in 2009 after negotiated guarantees on issues such as military neutrality and taxation. The Czech Republic, Poland and the United Kingdom secured various declarations and protocols addressing specific concerns during the ratification process. The treaty required unanimous ratification by all member states under their respective constitutional procedures; it entered into force on 1 December 2009 once all ratifications were completed. Supporters argued the treaty made the EU more efficient, democratically accountable and better equipped to act on the world stage. Critics contended that it centralized power, would further distance EU decision-making from national publics, or failed to address deeper questions about democratic legitimacy and social policy. The Lisbon Treaty’s reforms have had a lasting effect on how the EU operates: the permanent European Council president and the strengthened High Representative have become fixtures of EU diplomacy and governance, and the expanded use of qualified majority voting has reduced the frequency of national vetoes in many policy domains. The 2007 signing in Lisbon marked the formal political agreement on the text, but it was only the start of a complex ratification journey across diverse national political systems. By choosing amendment rather than constitution, EU leaders sought a pragmatic path to institutional reform that could survive domestic scrutiny, producing a legal structure that continues to shape the Union’s internal balance of power and external presence.