The great expansion of the European Union is understood to be the period 2003-2007. In 2003, the Treaty of Nice entered into force, which brought institutional changes to allow the functioning of the EU with a greater number of countries. In 2004, the Czech Republic Cyprus Slovenia Slovakia Estonia Hungary Latvia Lithuania Malta and Poland joined. Finally, in 2007, Romania and Bulgaria joined. In this way the EU of 15 became the EU of 27.