Croatia has signed a treaty to make it the 28th member of the European Union from mid-2013, becoming the EU's second ex-Yugoslav member after Slovenia.
EU Council President Herman Van Rompuy welcomed the "historic" treaty signed at the EU summit in Brussels after seven years of tortuous negotiations.
Territorial disputes with Slovenia and demands for the arrest of war crimes suspects had dogged the bid.
Serbia, meanwhile, is unlikely to get official candidate status until March.
According to a draft statement prepared for the summit, EU leaders will praise Serbia for democratic reforms but call for more evidence of progress in relations with Kosovo before making a final decision, EU diplomats said.
Officials, who spoke on customary condition of anonymity, told the Associated Press news agency Montenegro was likely to receive the green light to open accession negotiations next June.
Mr Van Rompuy said Croatia would be an "active observer" in all EU forums until it became a full member.
Croatian President Ivo Josipovic told the leaders of the 27 existing EU states: "Today Croatia is entering Europe, but more importantly Europe is entering Croatia."
Yugoslavia's split into independent states in the early 1990s sparked bloody conflicts, the last of which enveloped the Serbian region of Kosovo.
Belgrade has refused to recognise Kosovo's 2008 declaration of independence.