The European Union is based on the rule of law. This means that everything that it does is derived from treaties, which are agreed on voluntarily and democratically by all Member States. Previously signed treaties have been changed and updated to keep up with developments in society.

The main treaties are the following:

Treaty of Nice

The Treaty of Nice, signed on 26 February 2001, entered into force on 1 February 2003. It dealt mostly with reforming the institutions so that the Union could function efficiently after its enlargement to 25 Member States. The Treaty of Nice, the former Treaty of the EU and the Treaty of the EC have been merged into one consolidated version.

Treaty on European Union

The Treaty on European Union, which was signed in Maastricht on 7 February 1992, entered into force on 1 November 1993. 'The Maastricht Treaty changed the name of the European Economic Community to simply "the European Community". It also introduced new forms of co-operation between the Member State governments - for example on defence, and in the area of "justice and home affairs". By adding this inter-governmental co-operation to the existing "Community" system, the Maastricht Treaty created a new structure with three "pillars" which is political as well economic. This is the European Union (EU).

Merger Treaty

The Merger Treaty, signed in Brussels on 8 April 1965 and in force since 1 July 1967, which provided for a Single Commission and a Single Council of the then three European Communities.

Treaty establishing the European Coal and Steel Community

The Treaty establishing the European Coal and Steel Community (ECSC), which was signed on 18 April 1951 in Paris, entered into force on 23 July 1952 and expired on 23 July 2002.

The founding treaties have been amended on several occasions, in particular when new Member States acceded in 1973 (Denmark, Ireland, United Kingdom), 1981 (Greece), 1986 (Spain, Portugal), 1995 (Austria, Finland, Sweden), 2004 (the Czech Republic, Cyprus, Estonia, Hungary, Latvia, Lithuania, Malta, Poland, Slovakia and Slovenia), and 2007 (Bulgaria and Romania).