The ECB is responsible for the monetary policy of these 'euro zone' countries. However, the euro is not just another currency and its introduction will mean much more than simply dealing with one more foreign currency. Spring 1998 - 31 December 1998 Phase 1 started with the decision in May by EU governments on which countries qualify to join. The euro then became the legal currency in those countries. The European Central Bank became responsible for interest rates.
National currencies continue to exist in parallel to the euro, but changed in status.
No euro banknotes or coins are available, so national banknotes and coins are being used for all cash transactions. They will circulate alongside national currency banknotes and coins. By the end of the period, national banknotes and coins will be withdrawn from circulation. Old national banknotes will remain convertible into euro according to national practice. The Maastricht Treaty allows the UK to choose if and when it wants to join the single currency.