Eleven countries adopted the euro when it was launched in 1999, while six more countries have joined the bloc since then, most recently Estonia which joined at the beginning of 2011.
EU states obliged to join the Eurozone
Most of the European Union’s 27 member states still outside the euro are legally obliged to join the single currency when monetary conditions permit.
EU state with an opt-out on Eurozone participation
The exceptions are the UK and Denmark which have "opted out" of Eurozone membership.
Source: Al Jazeera