More than 300 million Europeans will have to get used to the new coins and notes quickly, and some are already finding the new money a bit strange.
"It doesn't look real. It's small, isn't it? It's a funny colour. It doesn't smell like money," said one visitor to Holland.
But others were giving the currency a go: in the small French Indian Ocean island of Reunion, the first sale using the money was a kilo of lychees!
Only three European Union countries have chosen not to use the new currency: UK, Denmark and Sweden. But as fireworks and street parties celebrated the change, these countries were encouraged to switch to the euro too.
Check out the links to the right of this story for your guide to the new currency as well as the chance to win a set of euro coins!