Romania and Bulgaria are preparing to celebrate their entry to the European Union from midnight tonight.
They will join the existing 25 members.
The President of the European Union, José Manuel Barroso, said it was an historic moment.
Their entries follow the EU's historic eastward enlargement in 2004.
The addition of the two Black Sea neighbours will raise the EU's membership to 27, almost half of them former communist countries once cut off from the West by the Iron Curtain.
Romania - the larger of the two - and Bulgaria will together boost the EU's population by 30 million, to 490 million, but will contribute just 1% to its total economic output.
Faster reforms, particularly the fight against corruption, finally secured their membership.
