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.
European Monetary System
The European Monetary System was no longer a functional arrangement in May 1998 as the member countries fixed their mutual exchange rates when participating in the euro. Its successor however, the ERM-II, was launched on 1 January 1999. In ERM-II the ECU basket was discarded and the new single currency euro has become an anchor for the other currencies participating in the ERM-II. Participation in the ERM-II is voluntary and the fluctuation bands remain the same as in the original ERM, i.e. +15 percent, once again with the possibility of individually setting a narrower band with respect to the euro. Denmark and Greece became new members.