The introduction of the euro as a cash currency completes the transition to the single currency in the 12 countries of the eurozone.
It has thrown up major challenges for the banking and retail sectors that could prove costly in the short-term.
The launch of the cash euro has wideranging effects on 300 million EU citizens from Dublin to Rome.
It should make it easier to travel and do business across Europe.
But it also throws up broader questions on the future politcal direction of the European Union.