In just over 100 days, the European Union will admit 10 new members, most of them former communist countries. The event has been hailed as an historic unification of the continent. But enlargement is also raising questions about how far the EU expansion should go. Much will depend on how the EU responds at the end of this year to Turkey's long-standing application.

This year's expansion of the EU will add 75 million people to the bloc's population, including 39 million from Poland alone. The other newcomers are the Czech Republic, Slovakia, Hungary and Slovenia, the three Baltic countries - Estonia, Latvia and Lithuania - and the Mediterranean islands of Cyprus and Malta.

The incoming members have gone to great lengths to transform themselves from centrally planned economies to free markets in barely a decade, and to incorporate hundreds of thousands of pages of EU law and rules onto their own statute books.

But uncertainties remain about how to reform the EU's own creaking institutions to ensure that its decision-making apparatus doesn't grind to a halt with 25 members.

An EU summit in December aimed at agreeing on an EU constitution collapsed amid acrimonious differences over members' proportional voting strength, and there is little prospect of a breakthrough anytime soon.

Now, the union must begin to grapple with the thorny question of what to do about Turkey, which is seeking a starting date to begin its own accession negotiations after first applying for membership 40 years ago. That decision is likely to prove as contentious as the constitutional dispute.

European Commission President Romano Prodi, who visited Turkey Thursday and Friday, has praised Prime Minister Recep Tayyip Erdogan's government for passing laws aimed at meeting EU standards of liberal democracy and respect for human rights. But Mr. Prodi says what is important is how those laws are implemented in the months ahead.

And then there is the question of Cyprus, whose northern part is still occupied by Turkish troops. Mr. Prodi says that finding a solution to Cyprus' division is not a precondition for Turkey starting accession talks, but that it would be helpful to Turkish aspirations for membership.

The EU has been ambivalent about Turkey's candidacy for years. Conservatives in such countries as France, Germany and Italy are uneasy about a Muslim state joining their club. But many Europeans are also worried about Turkey's size. By the time it would join, Turkey is projected to become the most populous member of the union and, therefore, would have more votes than any other EU country. And, being mostly poor and rural, it would get the biggest share of the EU's aid money.

Former French President Valery Giscard D'Estaing has argued that Turkey is not even European, since most of the country's land mass is in Asia. But what is Europe? And who is a European? Professor Jerome Sheridan, of American University's Brussels Center has one answer.

"Europe is a political concept. It is not a geographical concept," he said. "The accession of Cyprus to the European Union proves this once and for all. Cyprus' closest neighbors are Turkey and Lebanon. So, if Cyprus can be part of the European Union, you can no longer make the case that Turkey cannot be a part of the European Union."

Analyst Heather Grabbe, at London's Center for European Reform, says, asking where Europe's borders end is a bit like asking how long a piece of string is.

"The way the EU has tried to define it is more as a community of values. If you can sign up to our values, and you're reasonably close to Europe geographically, then, maybe, you can apply for membership," she said. "But this raises some very difficult dilemmas. Is Turkey European, for example, is a question that many Europeans still ask themselves, even though the EU has basically promised Turkey that it can join eventually. It has accepted it as a candidate. But then, there are other countries, like, say, Israel, or Morocco, where sometimes people talk about membership. Are they to be considered European? And, if, say, Turkey is accepted as European, then why not Ukraine as a member of the union?"

The European Commission has made it clear that, whatever the fate of Turkey's candidacy, the reunification of Europe will not be complete, until the Balkan countries join the Union. But countries on the southern and eastern Mediterranean rim are excluded both on geographical grounds, and because most fail the EU's strict standards on democracy and human rights.

Few people in Brussels see Russia ever joining the bloc. And, as for Ukraine, EU officials say its membership is only a distant possibility. That attitude may eventually change because newcomer Poland has been adamant about keeping the door open to its eastern neighbor and using the carrot of EU membership as a prod for economic and political reforms in Ukraine. On the other hand, Belarus and Moldova, two other former Soviet republics, are given little chance of becoming EU members in the foreseeable future.

East of Turkey lie the three republics of the Caucasus - Armenia, Georgia and Azerbaijan. EU interest in the area has been fueled by last year's "velvet revolution" in Georgia. Professor Sheridan says citizens of those countries that he has spoken with are absolutely convinced that they are Europeans, and should eventually be part of the EU, even though it may take them 20 years to join.

"Europe is a mentality. It's a frame of mind. It's a way of conducting international affairs. And when you join the European Union, you become like the countries that are in the West," he said. "So, in my opinion, Europe can continue to expand to the east, provided that the countries that are coming in look like the countries that are in the EU right now, in terms of their political culture. This is an extremely long-term process."

If Professor Sheridan is right, the EU will continue to expand, if it has the political will to do so, and as long as the countries that want to join the bloc behave like current EU members.