The EU was not always as big as it is nowadays: when European economic cooperation began in 1951, when the Paris Treatment was signed and which was the treaty that gave rise to what would later be the European Union, the only participating countries were Germany, Belgium, France, Italy, Luxemburg and the Netherlands. Over time many more countries joined. Throughout the years an enlargement of the union was produced, this means that since its founding in 1957, the EU has grown from 6 to 28 member countries which are independent sovereign European States.

Following the six pioneer countries, other Western European countries are joining since 1973. Later, after the collapse of their regimes in 1989, many former communist countries of Central and Eastern Europe were incorporated in two waves, between 2004 and 2007. With the accession of Croatia in 2013, the EU has twenty-eight member states.

During the last 50 years, the enlargement of the EU has boosted economic growth and consolidated democracy in countries emerging from a dictatorship.