Accession dates
1951 / 1957 Belgium, France, Germany, Italy, Luxembourg, Netherlands
1973 Denmark, Ireland, United Kingdom
1981 Greece
1986 Portugal, Spain 
1990 East Germany - German unification
1995 Austria, Finland, Sweden

The UK, Denmark and Sweden are not participating in the Euro currency.
 
 

Accession May 2004
Cyprus, Czech Republic, Estonia, Hungary, Latvia, Lithuania, Malta,
Poland, Slovak Republic, Slovenia

Accession planned for 2007
Bulgaria, Romania

Accession negotiations pending
Turkey