1957 The European Union started with just six countries, also called members.
1973 It expanded to nine countries, this is when the UK joined.
1981 Greece joined, so there were 10 members.
1986 Spain and Portugal joined to make it 12 EU countries.
1995 Three more countries joined making 15 members.
2004 Ten more countries joined, most of them in Eastern Europe. Taking the total to 25 members.
2007 Romania and Bulgaria joined, bringing the total to 27.
Lots of other countries are trying to get into the EU. Three of them - Croatia, the Former Yugoslav Republic of Macedonia (FYROM) and Turkey are all official candidates for membership.
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.