Adopting the single currency is the third stage of Economic and Monetary Union (EMU). Even though all Member States participate in EMU, not all of them use the euro: the United Kingdom and Denmark have opted out, while Bulgaria, Croatia, the Czech Republic, Hungary, Poland, Romania and Sweden are yet to adopt it, being formally obliged to do so under the Maastricht Treaty. Before this happens, they need to fulfil the convergence criteria of price stability, soundness and sustainability of public finances, durability of convergence and exchange rate stability. Furthermore, they must align their national legislation with the EU acquis on national central banks.
Adopting the euro Convergence criteria and state of play
Source European Parliament Think Tank - Jul 22, 15