Today consisting 28 member states the European Union is the biggest economic cooperation in the world. Following the WWII, in 1957 started with the European Coal and Steel Community with the Treaty of Rome, it first evolved into EEC (European Economic Community) and finally to its recent formation of "European Union" with Maastricht Treaty. The primary aim of the community was to end the struggles in Europe which harmed too much and caused two big world wars.
With customs union, there is free movement of the goods and labour between the member states. EU has its own legislative institutions like the European Parliament, The Council of the European Union, the European Commission, Court of Justice of European Union...etc. With well established economical and governmental institutions, functional regulations and monetary union (eurozone) today EU is the biggest rivals of the USA, China, Japan and India in economical and political terms which produces 20% of global gross domestic product. After the addition of Croatia in 2013, all member states can be stated as: Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, United Kingdom.
The bolded ones are also NATO members. Switzerland and Norway refused to enter to the EU mostly of economical concerns.
As specified above the last member was Croatia which was accepted in 2013. The question here now, will the European Union continue to expand or is this addition would be the last one? Has EU a policy towards Serbia, Macedonia, Azerbaijan and Turkey? Will these countries also be able to join the EU or wait some more time?