A European Constitution was first drafted in 2001 and came out of a debate on the future of Europe, at the Laeken European Council in December that year.
It's taken 8 years to make it fully operational. Here's more background on the past 8 years.
European Union leaders signed the bloc's first constitution in 2004.
But it needed to be ratified by all member states to enter into force.
On May 29th of 2005, the French public rejected the Constitution. And just three days later the Dutch also turned it down.
French Voter, said, "So far nothing is clear and the European laws must be entirely reviewed."
This called the future of the Constitution into question.
Following the period of reflection, the European Council meeting in June 2007 decided to start negotiations on a new reform treaty as a replacement. It aimed to amend the pre-existing EU treaties, and retain most of the reforms to the EU system that the constitution would have brought in.
This treaty was later named the Treaty of Lisbon.