The European Union has its roots in a 1951 agreement between six countries to work more closely together.
In 1973, the UK joined the EU along with two other countries.
There are now 25 members, after ten countries joined in 2004.
Many people agree the EU is now too big to make decisions the way it used to.
This is why they want a constitution or rule book to make some changes and set out clearly what all the countries can and cannot do.