All countries that join the EU will be expected to adopt the euro as soon as they can. But it will not generally be immediate. The paragraphs below are taken from the official EU website at europa.eu.int/comm/enlargement/faq/print-faq2.htm
|17. The euro|
Will the new members adopt the euro – and when?
According to the EU membership criteria, new members must be able to demonstrate the "ability to take on the obligations of membership including adherence to the aims of political, economic and monetary union." The future members are thus expected to adopt the euro when they are ready to do so, but not immediately upon accession.
The adoption of the common currency by a country, following accession, demands the respect of detailed conditions. This involves three stages:
So far, several of the applicant countries have made good progress towards these goals, at the same time strengthening their economic reforms.