Should Spain and Germany share the same money? On the surface, a single currency seems like a brilliant idea — no more exchange rate worries, easier travel, simpler trade. Yet the euro began as a dream of political unity and then hit a painful debt crisis, showing that a shared currency can also make economic problems much worse. This chapter explores when a group of countries really gains from giving up their own currencies, and how the euro area has struggled — and evolved — to meet that test.