There are a large number of criteria that potential European Union member countries must meet, but some key basic requirements are that the country be in Europe, that it has a free-market economy and a stable democracy. These terms are collectively called the "Copenhagen criteria."
As of February 2015, EU membership is open to any country in Europe that respects and is committed to promoting the organization's democratic values. The country must also be willing to comply with all of the EU's rules. The citizens of the country must consent by way of national parliament or referendum.
Applicant countries must have stable democratic institutions under the rule of law that guarantee human rights and protection of minorities. They must also have a market economy with the ability to handle market forces and competition within the EU, and must also be able to handle all the obligations of membership. Additional terms are added for countries from the Western Balkans.
There is a set of rules that covers 35 different policy fields that must be implemented by the member country without negotiation. Certain terms can be negotiated by the prospective member country during the application process, such as how much the new member pays into and receives from the general EU budget.