Enlargement of the eurozone – Wikipedia

The enlargement of the eurozone is an ongoing process within the European Union (EU).All member states of the European Union, except Denmark which negotiated opt-outs from the provisions, are obliged to adopt the euro as their sole currency once they meet the criteria, which include: complying with the debt and deficit criteria outlined by the Stability and Growth Pact, keeping inflation and …

https://en.wikipedia.org/wiki/Enlargement_of_the_eurozone