Eight former communist states -- the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Poland, Slovakia, and Slovenia -- as well as the two Mediterranean island states of Cyprus and Malta join the EU tomorrow.
But celebrations marking the bloc's expansion from 15 to 25 members start today, including midnight festivities scheduled that will include fireworks and the raising of star-studded blue EU flags.
One of the first official so-called "E-day" events takes place in the Polish capital of Warsaw, where German President Johannes Rau is to address the Polish parliament today.
An official welcoming ceremony will take place tomorrow in Dublin, Ireland, current holder of the rotating EU Presidency.
German Chancellor Gerhard Schroeder today said the EU will be fulfilling "a historic mission."
British Prime Minister Tony Blair said enlargement will give fresh impetus to "freedom and democracy in countries which for so long had neither."
And European Commission President Romano Prodi said the expansion ends the division of the European continent.
"Five decades after our great project of European integration began, the divisions of the Cold War are gone once and for all. And we live in a united Europe," Prodi said.
Yesterday, French President Jacques Chirac said EU enlargement is a "giant step" for Europe, and economic expansion means "everyone will be a winner."
EU expansion will turn the bloc into the world's largest free-trade area with 450 million citizens.
to see RFE/RL's "EU Expands Eastward" webpage.)