Ten countries have joined the euro since its launch in 1999
The euro began on 1 January 1999 with eleven members of the European Union. Since then ten more EU members have adopted it, each on 1 January of its year, from 2001 to 2026. No more than two have ever joined in the same year, and none has left, so the euro area now has 21 members. The first joiner arrived two years after the launch, in time for the first notes and coins in 2002. The other nine joined the EU in 2004, 2007 or 2013 and adopted the euro between 2007 and 2026. The list is closed for now: the ECB's June 2026 assessment found none of the five EU members it assessed ready to join.
Name them all first. Then try the harder part: name them by the period in which each joined.