Maastricht criteria

The euro convergence criteria (also known as the Maastricht criteria) are the criteria which European Union member states are required to meet to enter the third stage of the Economic and Monetary Union (EMU) and adopt the euro as their currency. The four main criteria, which actually comprise five criteria as the "fiscal criterion" consist of both a "debt criterion" and a "deficit criterion", are based on (ex article 121.1) of the Treaty on the Functioning of the European Union.

Full EMU membership is only open to EU member states. However, the European microstates of Andorra, Monaco, San Marino and the Vatican City, which due to their small size are not members of EU, have signed monetary agreements with EU which allow them officially to adopt the euro and issue their own variant of euro coins. These states had all previously used one of the eurozone currencies replaced by the euro, or a currency pegged to one of them. These states are not members of the eurozone and do not get a seat in the European Central Bank (ECB) or the Eurogroup.

As part of the EU treaty, all of the EU Member States are obliged to adhere to the Stability and Growth Pact (SGP), which as a framework to ensure price stability and fiscal responsibility, has adopted identical limits for governments budget deficit and debt as the convergence criteria. Due to the fact that several countries did not exercise a sufficient level of fiscal responsibility during the first 10 years of the euro's lifetime, two major SGP reforms were recently introduced. The first reform was the Sixpack which entered into force in December 2011, and it was followed in January 2013 by the even more ambitious Fiscal Compact, which was signed by 25 out of the then-27 EU member states.

...
Wikipedia