Membership criteria require that the candidate country must have achieved:
- stability of institutions guaranteeing democracy, the rule of law, human rights and respect for and protection of minorities;
- the existence of a functioning market economy as well as the capacity to cope with competitive pressure and market forces within the Union;
- the ability to take on the obligations of membership including adherence to the aims of political, economic and monetary union.
Consequently, all European Union member states are directly elected democracies that are considered to be "free" according to the criteria of Freedom House (with all of them being rated 1/1 except for Latvia with 2/1, Bulgaria and Greece with 1/2 and Romania with 2/2). As of 2008, all European Union member states are representative democracies; however, they do not all have the same political system, with most of the differences arising from different historical backgrounds.
Many of the European Union's neighbour states are not considered to be "free" by the same criteria. Most European states neighbouring the European Union are considered to be "free" or "partly free" — and improving — by Freedom House, with the notable exceptions of Armenia, Azerbaijan, Belarus and Russia. On the other hand, almost all of the states in North Africa and Southwest Asia that neighbour the European Union are not considered to be "free", with the notable exception of Israel.
At present, seven monarchies are members of the European Union: Belgium, Denmark, Luxembourg, the Netherlands, Spain, Sweden and the United Kingdom. All seven monarchies in the European Union are constitutional monarchies, which means that the monarch does not influence the politics of the monarch's country. Either the monarch is legally prohibited from doing so, or the monarch does not utilise the political powers vested in the office by convention. At the dawn of the 20th century, France was the only republic among the future members states of the European Union; the ascent of republicanism to the political mainstream only started at the beginning of the 20th century. Public opinion is currently strongly in favour of keeping the monarchy in the remaining seven monarchies in the European Union, and there is currently no ongoing campaign with popular support to abolish monarchy in any of them.
There are three types of government systems in European politics: in a presidential system, the president is the head of state and the head of government; in a semi-presidential system, the president and the prime minister share a number of competences; finally, in a parliamentary system, the president is a ceremonial figurehead who has few political competences. As with the definition of constitutional monarchies, sometimes the president does have non-ceremonial competences, but does not use them by constitutional convention; this is the case in Austria, for instance.
By definition, modern democratic constitutional monarchies are parliamentary, as there is no elected head of state who could assume non-ceremonial competences; of the twenty republican member states of the European Union, only one is a presidential republic (Cyprus) and one is a semi-presidential republic (France).
Most of the European Union's member states are unitary states, which means that most of the competences lie with the central government and only minor or local issues are within the authority of regional governments. However, three states are federations (Austria, Belgium and Germany) of states or regions with equal competences, and six other states have either devolved certain powers to special regions or are federacies (or both):
A further distinction is the number of chambers in the national legislature. While there had been legislatures with more than two chambers (tricameral and tetracameral ones), nowadays there are only unicameral and bicameral ones. There is no clear trend towards either model as of 2006, and there's also no real common factor which determines whether a country's legislature is unicameral or bicameral, except for the fact that federations and countries with strong regional differences or regional identities are normally bicameral to reflect the regions' interests in national bills. The states with the larger populations, from the Netherlands upwards, all have a bicameral system. Although there is a greater mix among the smaller states (some influenced by their federalist structure), the smallest states are on the whole unicameral.
In the member states of the European Union, if the parliament has only one chamber, it is wholly directly elected in all cases. If there are two chambers, the lower house is directly elected in all cases, while the upper house can be directly elected (e.g. the Senate of Poland); or indirectly elected, for example, by regional legislatures (e.g. the Federal Council of Austria); or non-elected, but representing certain interest groups (e.g. the National Council of Slovenia); or non-elected (though by and large appointed by elected officials) as a remnant of a non-democratic political system in earlier times (as in the House of Lords in the United Kingdom).