Which EU countries don't use the Euro?
Eurozone
Nine countries (Bulgaria, Croatia, Czech Republic, Denmark, Hungary, Poland, Romania, Sweden, and the United Kingdom) are EU members but do not use the euro. Before joining the eurozone, a state must spend two years in the European Exchange Rate Mechanism (ERM II). As of January 2017, only the National Central Bank (NCB) of Denmark participates in ERM II.
Economic history of Europe
The nations involved in the initial treaty were Austria, Belgium, Finland, France, Germany, Greece, Ireland, Italy, Luxembourg, the Netherlands, Portugal, and Spain. These nations agreed in principle to the European Monetary Union (EMU) in 1999 and installed the Euro as its currency on January 1, 2001. More European countries agreed to join the union in the following years; Slovenia (2007), Cyprus and Malta (2008), and Slovakia (2009). Countries are only allowed to begin utilizing the Euro when they have met certain requirements set about by the EMU. The criteria includes "a low and stable inflation, exchange rate stability and sound public finances." The reason for such criteria is because the best way to achieve a successful economy is by ensuring price stability.