Security is tight in Romania as dignitaries from NATO states turn out for the biggest ever summit of world leaders.
Streets are cordoned off and there is a heavy police presence in Bucharest.
The Romanian government mobilised thousands of police officers, fighter jets and snipers to maintain security.
The summit will host no fewer than 60 leaders, partners and aspiring new members.
Afghan President Hamid Karzai and U.N. Secretary-General Ban Ki-moon are also attending.
U.S. President George W. Bush gave a keynote speech ahead of the summit.
He urged NATO allies to send more troops to Afghanistan.
He says the alliance could not afford to lose its battle against Taliban insurgents and al Qaeda militants. Decisions on the membership hopes of Ukraine and Georgia are also on the agenda.
Germany, France and a handful of other western European allies say that now is not the time to offer them membership.