The European Union has undergone several enlargements since 1951, admitting new member states in 1973, 1981, 1986, 1995, 2004, and 2007. Key enlargements included the United Kingdom, Ireland, and Denmark joining in 1973; Greece joining in 1981; Spain and Portugal joining in 1986; Austria, Finland, and Sweden joining in 1995; and major eastern European countries like Poland, Hungary, and the Czech Republic joining in 2004. The most recent enlargement was in 2007 with the additions of Bulgaria and Romania.

1. EU Enlargements 1951-2007

2. 1951
Belgium
France
Germany
Italy
Luxembourg
the Netherlands

3. 1973
Denmark
Ireland
the United Kingdom

4. 1981
Greece

5. 1986
Spain
Portugal

6. 1995
Austria
Finland
Sweden

7. 2004
Cyprus
Czech Republic
Estonia
Hungary
Latvia
Lithuania
Malta
Poland
Slovakia
Slovenia

8. 2007
Bulgaria
Romania