Enlargement is the process whereby countries join the EU. Since it was founded in 1957, the EU has grown from 6 countries to 28.
Spreading prosperity and democracy
Over the past 50 years, widening EU membership has promoted economic growth and strengthened democratic forces in countries emerging from dictatorship.
Uniting East and West
The 6 founding members of the EU in 1957 were Belgium, France, Germany, Italy, Luxembourg and the Netherlands.
Several other West European countries joined after 1973.
Following the collapse of their regimes in 1989, many former communist countries from central and eastern Europe became EU members in 2 waves, between 2004 and 2007. In 2013, Croatia became the 28th country to join.
(source : http://europa.eu/pol/enlarg/index_en.htm)