Eight of the 10 new members are formerly communist states: the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Poland, Slovakia, and Slovenia.
The EU's current chairman, Irish Prime Minister Bertie Ahern, described the enlargement as the conclusive end to the artificial division of Europe, five decades after the Iron Curtain split the continent.
European Commission President Romano Prodi today welcomed the acceding countries, which join 15 existing EU states.
"It took courage, determination, and a lot of effort from the peoples and political forces in the new member states to get this far. It took vision and generosity from the peoples and the leaders in the current European Union," Prodi said.
Leaders from Austria, Italy, and Slovenia met on a mountain peak on their common border to mark the enlargement.
In the Hungarian capital of Budapest, an hourglass eight meters high was in place to mark the start of Hungary's time in the EU.
And while the Czechs held music festivals and prepared to shoot off fireworks at midnight, Czech Prime Minister Vladimir Spidla called the accession "the end of a long journey."
to see RFE/RL's "EU Expands Eastward" webpage.)