There are currently 17 member countries of the so called Eurozone. This is a name of the group of countries that have agreed on accepting the Euro as their official currency and so have discarded their old currency for the use of Euros instead. The process of introducing the Euro to these 17 countries has been long, and there are still countries that want to join the Eurozone.
The Euro started to be traded on 1 January 1999. However, there was no physical money of the currency at this point, but only computerized money in databases of banks. At this point, 11 countries had been allowed to start use the Euro and had so pegged their individual domestic currencies to the Euro. Exactly three year later, the first coins and notes of the Euro could be used for trade within these countries. The countries of the initial launch of the Euro included Austria, Belgium, Finland, France, Germany, Ireland, Italy, Luxemburg, Netherlands, Portugal, and Spain. These countries include some of the most influential economies in Europe, which lead to that the Euro quickly became accepted as an important currency in the world exchange market.
Although Greece was not an initial member of the Euro at the trade launch date in 1999, it still started trading the Euro in 2002, just like the original 11 member of the Eurozone countries. Greece was officially accepted in the new monetary union 1 January 2001, which meant that that first addition to the member countries was a fact.
After the launch of the currency had taken place and the value of the currency had stabilized, five years passed before any other country was let in to the Eurozone. In 2007 however, Slovenia had finally officially been accepted as a member after the long and complex admission process. Slovenia was the first country to join the Eurozone after the coins and notes had been issued in 2002, which made it more complicated to join that it was for Greece.
In 2008, two new countries made the Euro their official currency. Most large economies in Europe had already joined the Eurozone, which had left the opportunity for small countries to apply and actually be accepted. Malta and Cyprus, two island nations in the Mediterranean were the next two countries to be admitted.
Many people may not have seen it as particularly special when Slovakia joined the Eurozone in 2009. However, Slovakia was (and currently is) the only country that has adapted to the Euro that used to have a communist ruling power. This shows how far the development in Slovakia has come towards a democratic government.
As of now, the last addition to the Eurozone is Estonia that switched its official currency from having Kroons to Euros. This has taken Estonia many years, for example, their Euro coins were already designed and ready to be produced in 2004.