The foundation treaties of the European communities i.e. the European Coal and Steel Treaty, the European Atomic Energy Treaty and the European Economic Treaty together with the various annexes and protocols attached to them are a primary source of European Union law.

These treaties have been amended and supplemented on a number of occasions since the foundation of the European communities through the respective treaties of accession on the accession of the UK, Denmark and Ireland in 1973, Greece in 1981, Spain and Portugal in 1986, Finland, Austria and Sweden in 1995.

The Treaty of Accession 2003 signed in Athens on April 16, 2003, provided for the accession of the following countries to the European Union: Czech Republic, Estonia, Cyprus, Latvia, Lithuania, Hungary, Malta, Poland, Slovenia and Slovakia. More recently, the accession treaties for Bulgaria and Romania were signed on June 21, 2005 and these countries became members of the EU January 1 last year.

Besides enlargement, the treaties have also been amended by the Single European Act in 1986, the Treaty on European Union 1992, the Treaty of Amsterdam 1997, the Treaty of Nice 2001. The latter became effective throughout the European Union on February 1, 2003. Further amendments are expected with the Lisbon Treaty 2007 provided that it comes into force once ratified by all member states.

These treaties contain basic provisions on the European Union's objectives and organisation. They set the constitutional framework for the operation of the European Union which is administered by the union institutions.