As Croatia prepares to join the European Union on July 1, several other countries are still waiting in line -- most of them in the Balkans and the former Soviet Union.
Only three others -- Iceland, Montenegro and Turkey -- have begun membership talks but several are considered candidates and others still are seen as possible future bidders.
Here is a list of the current state of play for enlargement:
Turkey is the European Union's longest-lasting suitor -- it first applied in 1987 -- but talks have stalled in recent years mainly because of opposition from members like Germany.
Turkey, which actually began membership talks only in 2005, has also refused to normalise ties with Cyprus, whose northern half it occupied in 1974, leading to a long-running sovereignty dispute.
Hit by a series of unprecedented anti-government protests in past weeks and growing criticism of its human rights record, Turkey's membership now appears an even more distant prospect.
Membership negotiations began in 2010 but have so far skirted the most sensitive issues -- such as fisheries -- even though Iceland has already adopted a lot of EU regulation.
A new centre-right coalition came to power earlier this year and is less keen on joining than the previous government, saying it will first hold a referendum before continuing talks.
Iceland is also highly integrated in the European network as a member of the European Economic Area (EEA), the European Free Trade Association (EFTA) and the visa-free Schengen area.
Two-thirds of its foreign trade is with EU member states.
Negotiations began in 2012 and in April of this year Podgorica closed its second accession chapter out of 35, although key issues like reforming the justice system have still not been discussed.
Brussels has stressed that Montenegro has a European future but has urged the government to do far more to fight organised crime and corruption.
-- Former Yugoslav republic of MACEDONIA:
Skopje obtained the status of candidate in 2005 and the Commission recommended opening membership talks in 2009 but European leaders have still not given it the green light.
Greece is officially in favour but has said the country cannot use the title "Macedonia" as this is the name for a northern Greek province and has proposed it be called "North Macedonia."
Belgrade was granted candidate status last year and is hoping to get the go-ahead for the start of membership talks at a summit of EU leaders later this week.
The main remaining stumbling block to membership negotiations had been relations with Kosovo, which Belgrade normalised in an historic EU-brokered agreement signed in April.
Kosovo proclaimed its independence from Serbia in 2008 -- a status that Belgrade and some European countries like Spain still refuse to recognise.
Following the April normalisation deal with Serbia, the European Commission has recommended the opening of negotiations on a stability and association agreement with the EU.
Tirana applied in 2009. The main condition was that it hold elections in accordance with international norms, as well as clamp down on corruption and organised crime.
Sarajevo is behind its Balkan neighbours on ties with Europe. It has still has not amended its constitution in line with a request from the European Court of Human Rights which says it discriminates against Roma and Jewish minorities.
The constitution states only members of the three main communities -- Serbia, Croatian and Muslim -- can be elected to the upper chamber of parliament or the presidency.
-- Former Soviet republics:
Six former Soviet republics -- Armenia, Azerbaijan, Belarus, Georgia, Moldova and Ukraine -- are considered by the European Commission as possible future candidates for enlargement and are all members of the EU's Eastern Partnership.