There are now 15 European countries who are members of the eurozone, with a common currency, the euro, and a single interest rate set by the European Central Bank (ECB).
They make up 72% of the EU's GDP. How have they fared since the euro was created 10 years ago?
The euro was launched on 1 January 1999 on foreign exchange markets, and euro currencies replaced national currencies on 1 January 2002.
Despite increasing European integration, the eurozone countries still lag behind the US in the growth of productivity - the annual rate of growth of the output of the economy per worker.
This is a key measure of the standard of living, and there is broad agreement among economists that more economic reforms, especially in the labour market, are needed to raise EU productivity.
The eurozone economies have, however, managed to bring their budget deficits under control under the fiscal discipline of the Maastricht Treaty which set an upper limit of 3% of GDP.
The small overall budget deficit in the eurozone of just 0.6% has helped boost private investment and limited inflationary pressures.
It now puts many eurozone countries in a strong position to counter-act the effects of the global economic slowdown by spending more.
Since the euro has been launched, unemployment in the eurozone has gradually fallen and their have been 16 millions jobs created - a much better performance than the previous decade.
However, unemployment is still higher than in the US or the UK, especially among young people, and the economic slowdown in the next two years may lead to rising unemployment once again.
EURO CURRENCY MOVEMENTS
The euro has fluctuated against the US dollar since its launch in 1999.
Initially, it fell sharply to nearly $0.80, but in the last few years it has risen strongly against the dollar, breaching the $1.50 mark.
This has mainly been due to the weakness of the US dollar as the US trade deficit has soared and the US economy has moved towards recession.
The strong euro is making it difficult for European exporters and those eurozone manufacturers competing in internationally traded goods.
For a long time, the pound sterling tracked the value of the euro, and also rose strongly against the dollar.
More recently, the pound has broken free, and weakened against both the dollar and the euro.
This has made it more expensive for both UK tourists and importers.