The European Commission say they are "extremely satisfied" with the euro on the first day most banks and shops have been open to take it.
New wallets needed for new money
In Greece people are buying new wallets and are trying to remember how to use coins.
That's because one Greek drachma is worth less than a penny, and most shops didn't take or give out coins.
People have been very excited about the euro, with some even choosing to spend New Year's Eve queuing for the currency rather than celebrating the start of 2002.
Only three European Union countries have chosen not to use the new currency: UK, Denmark and Sweden.