The European Union (EU) consists of 28 member states. Each member state is party to the founding treaties of the union and thereby subject to the privileges and obligations of membership. Unlike members of most international organisations, the member states of the EU are subjected to binding laws in exchange for representation within the common legislative and judicial institutions. Member states must agree unanimously for the EU to adopt policies concerning defence and foreign policy. Subsidiarity is a founding principle of the EU.

In 1957, six core states founded the EU's predecessor, the European Economic Community (Belgium, France, Italy, Luxembourg, the Netherlands, and West Germany). The remaining states have acceded in subsequent enlargements. On 1 July 2013, Croatia became the newest member state of the EU. To accede, a state must fulfill the economic and political requirements known as the Copenhagen criteria, which require a candidate to have a democratic, free-market government together with the corresponding freedoms and institutions, and respect for the rule of law. Enlargement of the Union is also contingent upon the consent of all existing members and the candidate's adoption of the existing body of EU law, known as the acquis communautaire.

There is disparity in the size, wealth, and political system of member states, but all have de jure equal rights. In practice, certain states are considerably more influential than others. While in some areas majority voting takes place where larger states have more votes than smaller ones, smaller states have disproportional representation compared to their population.

No member state has withdrawn or been suspended from the EU, though some dependent territories or semi-autonomous areas have left. In June 2016, the United Kingdom held a referendum on membership of the EU, resulting in 51.89% of votes cast, being in favour of leaving. The UK government invoked Article 50 on 29 March 2017 to formally initiate the withdrawal process.

Acronyms have been used as a shorthand way of grouping countries by their date of accession.

According to the Copenhagen criteria, membership of the European Union is open to any European country that is a stable, free-market liberal democracy that respects the rule of law and human rights. Furthermore, it has to be willing to accept all the obligations of membership, such as adopting all previously agreed law (the 170,000 pages of acquis communautaire) and switching to the euro.^[33] To join the European Union, it is required for all member states to agree; if a single member state disagrees, the applying country is declined accession to the European Union. In addition to enlargement by adding new countries, the EU can also expand by having territories of member states, which are outside the EU, integrate more closely (for example in respect to the dissolution of the Netherlands Antilles) or by a territory of a member state which had previously seceded and then rejoined (see withdrawal below).

Enlargement is, and has been, a principal feature of the Union's political landscape.^[34] The EU's predecessors were founded by the "Inner Six", those countries willing to forge ahead with the Community while others remained skeptical. It was only a decade before the first countries changed their policy and attempted to join the Union, which led to the first skepticism of enlargement. French President Charles de Gaulle feared British membership would be an American Trojan horse and vetoed its application. It was only after de Gaulle left office and a 12-hour talk by British Prime Minister Edward Heath and French President Georges Pompidou took place that the United Kingdom's third application succeeded in 1970.^[35][36][37]

Applying in 1969 were the United Kingdom, Ireland, Denmark, and Norway. Norway, however, declined to accept the invitation to become a member when the electorate voted against it,^[38][39] leaving just the UK, Ireland, and Denmark to join.^[40] But despite the setbacks, and the withdrawal of Greenland from Denmark's membership in 1985,^[41] three more countries joined the Communities before the end of the Cold War.^[40] In 1987, the geographical extent of the project was tested when Morocco applied, and was rejected as it was not considered a European country.^[42]

The year 1990 saw the Cold War drawing to a close, and East Germany was welcomed into the Community as part of a reunited Germany. Shortly thereafter, the previously neutral countries of Austria, Finland, and Sweden acceded to the newly renamed European Union,^[40] though Switzerland, which applied in 1992, froze its application due to opposition from voters^[43] while Norway, which had applied once more, had its voters reject membership again in 1994.^[44] Meanwhile, the members of the former Eastern Bloc and Yugoslavia were all starting to move towards EU membership. Eight of these, plus Cyprus and Malta, joined in a major enlargement on 1 May 2004 symbolising the unification of Eastern and Western Europe in the EU.^[45] They were followed by Bulgaria and Romania in 2007 and then Croatia in 2013.^[46]

The EU has prioritised membership for the rest of the Western Balkans. Albania, North Macedonia, Montenegro, Serbia, and Turkey are all formally acknowledged as candidates, while Bosnia and Herzegovina and Kosovo are potential candidates.^[47] Turkish membership, pending since the 1980s, is a more contentious issue.^[48] Aside from the Cyprus dispute being a long-standing hurdle,^[49][50] relations between the EU and Turkey have become strained after several incidents, mostly concerning the 2016 Turkish coup d'état attempt, the Turkish referendum, and the resulting 2016–17 purges in Turkey.^[50] This has led to the European Parliament calling for a suspension of membership talks.^[51]

Each state has representation in the institutions of the European Union. Full membership gives the government of a member state a seat in the Council of the European Union and European Council. When decisions are not being taken by consensus, votes are weighted so that a country with a greater population has more votes within the Council than a smaller country (though the number of votes in relation to population is weighted disproportionately in favour of smaller member states). The Presidency of the Council of the European Union rotates among each of the member states, allowing each state six months to help direct the agenda of the EU.^[52][53]

Similarly, each state is assigned seats in Parliament according to their population (again, with the smaller countries receiving more seats per inhabitant than the larger ones). The members of the European Parliament have been elected by universal suffrage since 1979 (before that, they were seconded from national parliaments).^[54][55]

The national governments appoint one member each to the European Commission (in accord with its president), the European Court of Justice (in accord with other members) and the European Court of Auditors. Historically, larger member states were granted an extra Commissioner. However, as the body grew, this right has been removed and each state is represented equally. The six largest states are also granted an Advocates General in the Court of Justice. Finally, the Governing Council of the European Central Bank includes the governors of the national central banks (who may or may not be government appointed) of each euro area country.^[56]

The larger states traditionally carry more weight in negotiations, however smaller states can be effective impartial mediators and citizens of smaller states are often appointed to sensitive top posts to avoid competition between the larger states. This, together with the disproportionate representation of the smaller states in terms of votes and seats in parliament, gives the smaller EU states a greater clout than normally attributed to a state of their size. However most negotiations are still dominated by the larger states. This has traditionally been largely through the "Franco-German motor" but Franco-German influence has diminished slightly following the influx of new members in 2004 (see G6).^[57]

While the member states are sovereign, the union partially follows a supranational system that is comparable to federalism. Previously limited to European Community matters, the practice, known as the "community method", is currently used in most areas of policy. Combined sovereignty is delegated by each member to the institutions in return for representation within those institutions. This practice is often referred to as "pooling of sovereignty". Those institutions are then empowered to make laws and execute them at a European level.

If a state fails to comply with the law of the European Union, it may be fined or have funds withdrawn.

In contrast to other organisations, the EU's style of integration has "become a highly developed system for mutual interference in each other's domestic affairs".^[58] However, on defence and foreign policy issues (and, pre-Lisbon Treaty, police and judicial matters) less sovereignty is transferred, with issues being dealt with by unanimity and co-operation. Very early on in the history of the EU, the unique state of its establishment and pooling of sovereignty was emphasised by the Court of Justice:^[59]

The question of whether EU law is superior to national law is subject to some debate. The treaties do not give a judgement on the matter but court judgements have established EU's law superiority over national law and it is affirmed in a declaration attached to the Treaty of Lisbon (the European Constitution would have fully enshrined this). Some national legal systems also explicitly accept the Court of Justice's interpretation, such as France and Italy, however in Poland it does not override the national constitution, which it does in Germany. The exact areas where the member states have given legislative competence to the EU are as follows. Every area not mentioned remains with member states.^[61]

As a result of the European sovereign debt crisis, some eurozone states required a bailout from their fellow members via the European Financial Stability Facility and European Financial Stability Mechanism (to be replaced by the European Stability Mechanism from 2013), but this came with conditions. As a result of the Greek government-debt crisis, Greece accepted a large austerity plan including privatisations and a sell off of state assets in exchange for their bailout. To ensure that Greece complies with the conditions set by the European troika (ECB, IMF, Commission), a "large-scale technical assistance" from the European Commission and other member states has been deployed to Greek government ministries. Some, including the President of the Euro Group Jean-Claude Juncker, state that "the sovereignty of Greece will be massively limited."^[62][63][64] The situation of the bailed out countries (Greece, Portugal and Ireland) has been described as being a ward^[65][66] or protectorate^[64][67][68] of the EU with some such as the Netherlands calling for a formalisation of the situation.^[69]

EU integration is not always symmetrical, with some states proceeding with integration ahead of hold-outs. This comes in two forms; a faster integrated core where some states forge ahead with a new project, and opt-outs where a few states are excused from normal integration. The notion of multi-speed integration is anathema to some, including President Juncker, who see it as divisive to the European project and others, such as the less-integrated states, who feel they would be left behind. It is however supported by others, such as President Macron, to move forward in integration faster.^[70][71]

There are several different forms closer integration both within and outside the EU's normal framework. The main mechanism is enhanced cooperation where nine or more states can use EU structures progress in a field that not all states are willing to partake in. One example of this is the European Public Prosecutor.^[72] A similar mechanism is Permanent Structured Cooperation in Defence, where the majority of EU states work in a flexible manner on defence cooperation. Other projects, such as the European Fiscal Compact, operate between EU members but as a separate intergovernmental treaty outside of the official EU structures.^[73]

A number of states are less integrated into the EU than others. In most cases this is because those states have gained an opt-out from a certain policy area. The most notable is the opt-out from the Economic and Monetary Union, the adoption of the euro as sole legal currency. Most states outside the Eurozone are obliged to adopt the euro when they are ready, but Denmark and the United Kingdom have obtained the right to retain their own independent currencies.

Ireland and the United Kingdom also do not participate in the Schengen Agreement, which eliminates internal EU border checks. Denmark has an opt out from the Common Security and Defence Policy; Denmark, Ireland and the UK have an opt-out on police and justice matters and Poland and the UK have an opt out from the Charter of Fundamental Rights.^[74]

There are a number of overseas member state territories which are legally part of the EU, but have certain exemptions based on their remoteness. These "outermost regions" have partial application of EU law and in some cases are outside of Schengen or the EU VAT area—however they are legally within the EU.^[75] They all use the euro as their currency.

Entry to the EU is limited to liberal democracies and Freedom House ranks all EU states as being totally free electoral democracies.^[77] All but 4 are ranked at the top 1.0 rating.^[78] However, the exact political system of a state is not limited, with each state having its own system based on its historical evolution.

Half of member states—14 out of 28—are parliamentary republics, while seven states are constitutional monarchies, meaning they have a monarch although political powers are exercised by elected politicians. Most republics and all the monarchies operate a parliamentary system whereby the head of state (president or monarch) has a largely ceremonial role with reserve powers. That means most power is in the hands of what is called in most of those countries the prime minister, who is accountable to the national parliament. Of the remaining republics, five operate a semi-presidential system, where competencies are shared between the president and prime minister, while one republic operates a presidential system, where the president is head of state and government.

The EU is divided between unicameral (single chamber) and bicameral (dual chamber) parliaments, with 15 unicameral national parliaments and 13 bicameral parliaments. The prime minister and government are usually directly accountable to the directly elected lower house and require its support to stay in office—the exception being Cyprus with its presidential system. Upper houses are composed differently in different member states: it can be directly elected like the Polish senate, indirectly elected, for example, by regional legislatures like the Federal Council of Austria, unelected, but representing certain interest groups like the National Council of Slovenia, unelected (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). Most (though not all) elections in the EU use some form of proportional representation. The most common type of proportional representation is the party-list system.

There are also differences in the level of self-governance for the sub-regions of a member state. Most states, especially the smaller ones, are unitary states; meaning all major political power is concentrated at the national level. 10 states allocate power to more local levels of government. Austria, Belgium and Germany are full federations, meaning their regions have constitutional autonomies. Denmark, Finland, France and the Netherlands are federacies, meaning some regions have autonomy but most do not. Spain and Italy have system of devolution where regions have autonomy, but the national government retains the right to revoke it. The United Kingdom has a system of asymmetric devolution, whereby Scotland, Wales, and Northern Ireland enjoy a degree of self-government.^[79]

States such as France have a number of overseas territories, retained from their former empires. Some of these territories such as French Guiana are part of the EU (see outermost regions, above) while others are related to the EU or outside it, such as the Falkland Islands.

The Lisbon Treaty made the first provision of a member state to leave. The procedure for a state to leave is outlined in TEU Article 50 which also makes clear that "Any Member State may decide to withdraw from the Union in accordance with its own constitutional requirements". Although it calls for a negotiated withdrawal between the seceding state and the rest of the EU, if no agreement is reached two years after the seceding state notifying of its intention to leave, it would cease to be subject to the treaties anyway (thus ensuring a right to unilateral withdrawal).^[80] There is no formal limit to how much time a member state can take between adopting a policy of withdrawal, and actually triggering Article 50.

In a non-binding referendum in June 2016—the result of which the government promised to implement—the United Kingdom voted to withdraw from the EU. Termed "Brexit", this has become government policy under Prime Minister Theresa May. UK government triggered Article 50 on 29 March 2017.^[81] Once triggered, formal talks could begin but there is no certainty of a deal and some EU officials are preparing to deal with a situation where no deal is reached after the two-year limit.^[82]

Prior to 2016, no member state had ever voted to withdraw. However, French Algeria, Greenland and Saint-Barthélemy did cease being part of the EU (or its predecessor) in 1962, 1985, and 2012, respectively, due to status changes. The situation of Greenland being outside the EU while still subject to an EU member state had been discussed as a template for the pro-EU regions of the UK remaining within the EU or its single market.^[83]

Beyond the formal withdrawal of a member state, there are a number of independence movements such as Catalonia or Flanders which could result in a similar situation to Greenland. Were a territory of a member state to secede but wish to remain in the EU, some scholars claim it would need to reapply to join as if it were a new country applying from scratch.^[84] However, other studies claim internal enlargement is legally viable if, in case of a member state dissolution or secession, the resulting states are all considered successor states.^[85] There is also a European Citizens' Initiative that aims at guaranteeing the continuity of rights and obligations of the European citizens belonging to a new state arising from the democratic secession of a European Union member state.^[86]

There is no provision to expel a member state, but TEU Article 7 provides for the suspension of certain rights. Introduced in the Treaty of Amsterdam, Article 7 outlines that if a member persistently breaches the EU's founding principles (liberty, democracy, human rights and so forth, outlined in TEU Article 2) then the European Council can vote to suspend any rights of membership, such as voting and representation. Identifying the breach requires unanimity (excluding the state concerned), but sanctions require only a qualified majority.^[87]

The state in question would still be bound by the obligations treaties and the Council acting by majority may alter or lift such sanctions. The Treaty of Nice included a preventative mechanism whereby the Council, acting by majority, may identify a potential breach and make recommendations to the state to rectify it before action is taken against it as outlined above.^[87] However, the treaties do not provide any mechanism to expel a member state outright.^[80]

There are a number of countries with strong links with the EU, similar to elements of membership. Following Norway's decision not to join the EU, it remained one of the members of the European Economic Area which also includes Iceland and Liechtenstein (all former members have joined the EU, and Switzerland rejected membership). The EEA links these countries into the EU's market, extending the four freedoms to these states. In return, they pay a membership fee and have to adopt most areas of EU law (which they do not have direct impact in shaping). The democratic repercussions of this have been described as "fax democracy" (waiting for new laws to be faxed in from Brussels rather than being involved in drafting them).^[88]

A different example is Bosnia and Herzegovina, which has been under international supervision. The High Representative for Bosnia and Herzegovina is an international administrator who has wide-ranging powers over Bosnia and Herzegovina to ensure the peace agreement is respected. The High Representative is also the EU's representative, and is in practice appointed by the EU. In this role, and since a major ambition of Bosnia and Herzegovina is to join the EU, the country has become a de facto protectorate of the EU. The EU appointed representative has the power to impose legislation and dismiss elected officials and civil servants, meaning the EU has greater direct control over Bosnia and Herzegovina than its own states. Indeed, the state's flag resembles the EU's flag according to some observers.^[89]

In the same manner as Bosnia and Herzegovina, Kosovo is under heavy EU influence, particularly after the de facto transfer from UN to EU authority. In theory Kosovo is supervised by EU missions, with justice and policing personal training and helping to build up the state institutions. However the EU mission does enjoy certain executive powers over the state and has a responsibility to maintain stability and order.^[90] Like Bosnia, Kosovo has been termed an "EU protectorate".^[91][92][93]

However, there is also the largely defunct term of associate member. It has occasionally been applied to states which have signed an association agreement with the EU. Associate membership is not a formal classification and does not entitle the state to any of the representation of free movement rights that full membership allows. The term is almost unheard of in the modern context and was primarily used in the earlier days of the EU with countries such as Greece and Turkey. Turkey's association agreement was the 1963 Ankara Agreement, implying that Turkey became an associate member that year.^[94][95] Present association agreements include the Stabilisation and Association Agreements with the western Balkans; these states are no longer termed "associate members".