Huge street parties, sparkling firework displays and joyful concerts have heralded the biggest expansion in the history of the European Union.
Hundreds of thousands across Europe celebrated the event
The 15 old members welcomed in Cyprus, the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Malta, Poland, Slovakia and Slovenia at midnight.
Leaders of all the members will later join Irish Prime Minister Bertie Ahern for a flag-raising ceremony in Dublin.
The expansion to 25 nations makes the EU the world's biggest trading bloc.
The EU now has a combined population of 455 million.
Polish President Aleksander Kwasniewski said a dream had become reality.
Hundreds of thousands packed city squares in the newcomer states to see the fireworks on the stroke of midnight and hear Beethoven's Ode to Joy - the EU's official anthem.
Ireland, which holds the EU's revolving presidency, will host a Day of Welcomes including official ceremonies on Saturday but its capital Dublin also joined in the fun at midnight, with a massive "Stars of the Sea" display of rockets on the city's Sandymount Strand.
The BBC's Tim Franks notes some enthusiasts are describing the enlargement as a millennial event, comparable to the creation of great empires.
Many of the 10 new members are poor with young institutions of state and they see the promise of Europe-wide stability and eventual enrichment.
This is a hugely significant day for Europe, our correspondent says, but it is nowhere near the end of the story.
In the existing member states, there is more uncertainty over immigration, over the new balance of forces within the EU and over whom the club should admit next.
Eight of the new members are former communist states, joining the Western club only 15 years after most of them emerged from years of Soviet domination. Some were not even separate countries until just over a decade ago.
Now all their governments have satisfied Brussels that their economies, legal systems and democracies are ready for EU membership.
The foreign ministers of Poland and Germany marked the event with an embrace
The other two new members - Malta and Cyprus - are Mediterranean islands although Cypriot membership is being overshadowed by the exclusion of the island's Turkish Cypriot part.
One of the fathers of European reunification, Helmut Kohl, spoke through tears when he addressed thousands at a ceremony in the German town of Zittau, which borders both Poland the Czech Republic.
"The message is there will never again be war in Europe," the former German chancellor said.
Marek Wos, a 40-year-old Polish businessman attending the celebrations in Warsaw, said it was a good day for his country.
"We will no longer be second-class people from a second-class country," he said.
On Saturday afternoon all 25 leaders are expected to join Irish Prime Minister Bertie Ahern for a flag-raising ceremony in Dublin.
Security for the formal ceremonies is expected to be particularly tight. Irish media report that police leave has been cancelled and 4,000 officers will be patrolling the streets.
Some 2,500 soldiers are also said to be on stand-by.
Anti-globalisation activists are expected to hold protests.