On Saturday 1 May 2004 the EU grew from 15 to 25 members.
This is the biggest expansion of the Union since it started.
There were a lot of news stories in the UK about whether or not the enlargement would benefit the UK.
Some people were worried that workers in the new member states would move to Britain to get better paid jobs.
How the EU has grown:
1957 It started with six members.
1973 It expanded to 9 countries, this is when the UK joined.
Greece joined, so there were 10 members.
Spain and Portugal joined to make it 12 EU countries.
1995 Three more countries joined making the current 15 members.
2004 Ten more countries joined. Most of them are from Eastern Europe. There are now 25 members.
Talks will begin about Turkey joining.
Two other countries are due to join bringing the total to 27
How do you join?
Countries have to prove certain things.
1. They must show that they treat their people fairly, respect their human rights and allow them to vote in elections.
2. They must show that their economies are properly run. That means the government is sensible about the amount of money it spends and does not interfere too much in the way people do business.
3. Countries may have to make changes to their laws so that they don't clash with the laws of the EU.