BRUSSELS (Reuters) - The European Commission publishes on Tuesday its annual progress reports on countries aspiring to join the European Union.
About a dozen states in Europe hope to become members of the wealthy 27-state bloc in the coming decades, but their success depends on extensive democratic and market reforms and on the EU's ability to shake off reluctance to grow.
Since admitting 12 mostly eastern European countries in the last decade, the EU has taken a slower approach to enlargement, shifting its focus to institutional reforms and efforts to solve its economic problems.
Below is an explanation of the bloc's enlargement policy and issues affecting its implementation.
Any country in Europe that respects the principles of the rule of law, liberty, democracy, human rights and fundamental freedoms may apply to become a member.
There are four official candidates: Croatia, Iceland, Macedonia and Turkey. Albania, Montenegro and Serbia have also applied, but have yet to earn candidate status. Also hoping to join are Bosnia and Kosovo.
The EU also has a "neighbourhood policy" aimed at forging closer ties with other countries in eastern Europe such as Ukraine, Belarus and Moldova and in the Caucasus. In contrast to the western Balkans, these countries have never been explicitly promised membership.
Once countries gain candidate status, they hold negotiations with the bloc on more than 30 policy areas, known as "chapters" in a process designed to align their legislation with EU rules.
The EU's executive, the European Commission, is pushing to keep the enlargement process on track in hopes of overcoming the "enlargement fatigue" that has set in the bloc.
It argues new members can strengthen the bloc in a number of ways, from ensuring economic competitiveness to bolstering the EU's clout on the world stage.
Growing eastward, it says, allows the EU to develop better transport corridors for trade, have more impact over climate change because EU rules force newcomers to clean up dirty industries, and better control over energy routes.
The membership of Turkey could give the bloc more influence over the Middle East and the energy-rich Caucasus, the Commission argues.
Closer ties with western Balkans are important for the bloc's security, the Commission says, because they ensure better control over major drug routes from Asia to Europe. Bringing in former Yugoslav states also could allow the bloc to curb illicit trafficking in arms and people.
Economically, new members mean more trade and younger populations, vital to ensuring long-term competitiveness. Turkey's economy in particular has been growing at a much faster rate than those within the EU.
According to Commission data, trade between "old" and "new" members has tripled from around 150 billion euros ($209 billion) to nearly half a trillion euros in the decade up to 2008.
The enlargement policy has several strong supporters among EU governments, notably Britain, Sweden and the new members in eastern Europe. But many states want to take the process slowly.
France, Germany, Austria and the Netherlands have expressed concerns over the cost of allowing in new members at a time when the bloc is still struggling to rise from recession and arguing over how to fund future financial crises like the one that hit Greece this year.
In addition to budget costs, there are also worries about selling the idea of enlargement to a European public concerned about job losses and competition for employment from lower-paid eastern neighbours.
France and Germany, in particular, are concerned about bringing Turkey into the fold. If admitted, Turkey would be the second-largest EU state in terms of population and would have substantial voting power in the bloc.
Turkey would be the first mainly-Muslim member of an otherwise Christian bloc, in which some political parties have tapped into public concern about the assimilation of Muslim migrants. The Commission and most mainstream European parties say religion should play no part in evaluating Turkey's bid.
Turkey's bid also would require it to resolve a territorial dispute with EU member Cyprus.
Editing by Rex Merrifield and Peter Graff