Every action taken by the EU is founded on treaties that have been approved voluntarily and democratically by all EU Member States. A treaty represents a binding agreement between EU Member States. It sets out EU objectives, rules for EU institutions, how decisions are made and the relationship between the EU and its Member States. Treaties are amended to make the EU more efficient and transparent, to prepare for new member countries and to introduce new areas of cooperation. Under the treaties, EU institutions can adopt legislation, which the member countries then implement. Currently valid founding treaty is:
Treaty of Lisbon (2009)
Purpose of this most recent treaty was to make the EU more democratic, more efficient and better able to address global problems. It has introduced several major changes: more power was assigned to the European Parliament, voting procedures in the Council have been changed, citizens' initiative has been introduced, as well as permanent president of the European Council and new High Representative for Foreign Affairs. The Lisbon Treaty clarified separation of powers by introducing categorization of competences, those that belong to the EU, those that belong to Member States and shared.
Previous EU founding treaties are:
The Treaty establishing a Constitution for Europe (2004)
This Treaty had similar aims as the Lisbon Treaty. It was signed but never ratified since ratification process was brought to an end by rejection of French and Dutch voters in 2005.
You can read more about the Treaty establishing a constitution for Europe here.
Treaty of Nice (2003)
Looking at the forthcoming enlargement in 2004 the Treaty was to reform the institutions so that the EU could function efficiently after reaching 25 member countries. Methods for changing the composition of the Commission have been changed as well as structure of the voting system in the Council.
Treaty of Amsterdam (1999)
This Treaty also had the purpose to prepare the Union for the arrival of future Member States. It has amended, renumbered and consolidated EU and EEC treaties. Also, decision-making has been made more transparent by increasing use of the co-decision voting procedure.
Treaty on European Union - Maastricht Treaty (1993)
European Union was established and the co-decision procedure introduced thus giving Parliament more power in decision-making. Elements of a political union like citizenship and common foreign policy and internal affairs policy have been introduced. Also, Treaty had purpose to prepare conditions for European Monetary Union.
Single European Act (1986)
Preparing the institutions for accession of Portugal and Spain's and speeding up decision-making in preparation for the single market. It was harder for a single country to veto proposed legislation since qualified majority voting in the Council has been modified.
You can read more about the Single European Act here.
Merger Treaty - Brussels Treaty (1967)
Its purpose was to streamline the European institutions by creating single Commission and a single Council to serve former European Communities (EEC, Euratom, ECSC). It was repealed by the Treaty of Amsterdam.
You can access full text (in French) of the Merger Treaty by submitting personal e-mail address here.
Treaties of Rome - EEC and EURATOM treaties (1958)
These Treaties set up the European Economic Community (EEC) and the European Atomic Energy Community (Euratom). General economic cooperation has been introduced and therefore European integration extended.
Treaty establishing the European Coal and Steel Community (1952)
Starting point of European integrations. It started with creation of cooperation and interdependence in coal and steel industry. World War II ended several years ago and this initiative aimed at preventing future wars on the European continent since coal and steel were essential for weapons manufacturing. It expired in 2002.
You can read more about the Treaty establishing the European Coal and Steel Community here.