Romanians and Bulgarians planned all-night street parties to see in the New Year and mark their countries' accession to the European Union on January 1st.
They join a club that is increasingly hesitant to admit new members after its 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.
As they joined the EU ranks, some west Europeans were worried that the bloc might have overstretched itself. They feared further enlargement could hurt their job markets, even raising the crime rate if, for example, drug smuggling and people trafficking, rife in the Black Sea region, spread to the West.
Other EU hopefuls in the Balkans and further south, such as former Yugoslav states, Albania and Turkey, may have to wait.
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 one per cent to its total economic output. The EU's borders will stretch from the Atlantic and Baltic in the west and north to the Black Sea in the southeast.
Street parties were planned throughout Romania and Bulgaria, which missed out in the first wave of the bloc's eastward expansion in 2004. Faster reforms, particularly the fight against corruption, finally secured their membership. "Tonight we write history," the Romanian newspaper Evenimentul Zileisaid in the headline on its front page, printed all in blue with yellow stars.
Romanians and Bulgarians were relieved to join the bloc, despite the threat of further delays, and hoped Brussels would help prod their governments to continue democratic reforms and anti-corruption efforts. The two countries, which have the lowest per capita incomes in the bloc, are plagued by outdated technology, shabby infrastructure and poor education. Both economies are growing fast, however.
Political analysts expected membership and EU funds to sustain robust growth, but progress to be hampered by inept administration, graft and crime gangs. Already, the momentum of reform is declining, many infrastructure projects remain on the drawing board and absorption of EU funds is expected to be slow.
Analysts were also concerned about weaknesses in Bulgaria, which is expected to lag behind Romania in improving its administration and fighting what Brussels once called "a climate of impunity" among corrupt officials.
Critics in the West fear difficulties at home could produce a new wave of immigration after the exodus of Poles and other central Europeans in the wake of the 2004 expansion. Most of the older EU nations have refused to open their borders completely to Bulgarians and Romanians but many central European countries welcome new entrants to their job markets.