The new EU will stretch more than 3,000 kilometres from east to west, spanning three time zones and a dramatic array of climates. The size of an EU state will range from a few hundred thousands to nearly 90 million, while the average yearly income in one country will not be much more than the typical monthly wage in another.
In a Europe of such contrasts, BBC News Online asked those living in the extremities to discuss their hopes and fears for the future in an enlarged EU.
Name: Kari Parkkonen
Occupation: Senior social worker
Country: Finland, the new EU's northernmost state.
I think that enlargement is a very positive thing. The bad thing is that Estonia, Latvia and Lithuania are joining at a time when their living standards are still very low. That means that many people in those countries will migrate to other EU countries.
But I think it is definitely necessary for all of Europe to be united in the future, because the USA is so powerful and we need some balance. We don't really know what will happen next, what president they will elect and what other Arab country they might decide to invade.
How do you feel about living in the northernmost EU country?
I like Finland! It's definitely my favourite country, so the fact that it's the northernmost one is not a problem at all. We have everything here, and don't live so far away from the rest of Europe. We can always catch a plane and go there if we feel the need.
I also like the fact that we have four seasons with a cold, snowy winter when you can go skiing. The only problem is that we are a bit far away from Brussels and the EU institutions.
I think that in Brussels they don't really know what kind of problems we have. For example, farming is very different here: we have long winters with a lot of snow, and growing wheat and vegetables is much more expensive then elsewhere.