The European Union, or EU, describes itself as a family of democratic European countries, committed to working together for peace and prosperity.
The organisation oversees co-operation among its members in diverse areas, including trade, the environment, transport and employment.
On 1 May 2004 the EU took in 10 new members, most of them former communist countries, in a huge step along the road towards dismantling the post-World War II division of Europe.
The new joiners were the Czech Republic, Cyprus, Estonia, Hungary, Latvia, Lithuania, Malta, Poland, Slovakia and Slovenia.
However, plans to introduce a constitution - intended to ensure the smooth running of the enlarged EU - faltered repeatedly at various national referendums until the revised "Lisbon" reform treaty was adopted. It came into force in December 2009.
Over half a century earlier, it was the devastation caused in Europe by World War II which underlay the feeling imperative to build international relationships to guard against any such catastrophe recurring.
French statesmen Jean Monnet and Robert Schuman are regarded as the architects of the principle that the best way to start the European bonding process was by developing economic ties.
This philosophy was the foundation for the Treaty of Paris of 1951. It established the European Coal and Steel Community (ECSC) which was joined by France, Germany, Italy, the Netherlands, Belgium and Luxembourg.
Under the Treaty of Rome which came into force in 1958, these six countries founded the European Economic Community and European Atomic Energy Community to work alongside the ECSC.
In 1967 the three communities merged to become collectively known as the European Communities (EC) whose main focus was on cooperation in economic and agricultural affairs.
Denmark, Ireland and the UK became full EC members in 1973, Greece joined in 1981, Portugal and Spain in 1986, Austria, Finland and Sweden in 1995.
Maastricht and beyond
The Treaty on European Union, signed at Maastricht in 1991, formally established the European Union as the successor to the EC.
At the same time, Maastricht expanded the concept of European union, introducing a common foreign and security policy and moving towards an EU coordinating policy on asylum, immigration, drugs and terrorism.
EU citizenship was brought into being for the first time, allowing people from member countries to move freely between member states. A new Social Chapter - from which the UK opted out - laid down EU policies on workers' rights and other social issues.
Crucially, Maastricht established the timetable for economic and monetary union and specified the criteria for countries wanting to join.
The subsequent Stability and Growth Pact tightened up these criteria, stressing strict fiscal discipline and coordination and laying down penalties for failing to control budget deficits.
The single European currency, the euro, was officially adopted by 11 member states in 1999. Greece, which took longer to meet convergence criteria, joined two years later. Denmark, Sweden and the UK chose not to join.
The failure of many eurozone countries to stick to the self-imposed rules on government debt triggered a major financial crisis in 2009.
Jointly with the IMF, the EU approved a series of multi-billion-euro bailouts to contain the problem, starting with 110bn euros for Greece in May 2010, 85bn euros for Ireland in November and 78bn euros for Portugal in May 2011.
This and other emergency measure failed to restore confidence as the indebtedness of Spain, Italy and Cyprus also prompted concern. Rifts opened up about between richer and poorer countries how to deal with the crisis, with Germany emerging as the the chief advocate of austerity.
In December 2013, EU leaders approved a set of rules for managing the closure of failing eurozone banks, intended to minimise the need for taxpayer-funded bailouts.
Other key issues
Supporters of the 2004 influx of new member states saw enlargement as the best way of building economic and political bonds in Europe and end past divisions of the past.
Critics voiced concern than expansion would deliver new economic burdens, make decision-making harder and open up richer members to an influx of immigrants from former communist states.
Bulgaria and Romania joined in January 2007, Croatia in mid-2013, and in 2009 Serbia submitted a formal application to join. Talks about Turkey's possible accession began in October 2005.
To prepare for the first big wave of enlargement, a convention was established in 2002 to draft an EU constitution to streamline and replace the complex array of treaties and agreements governing the union, and more clearly define its powers
A draft charter agreed at an EU summit in June 2004 for ratification by individual member states - either by parliament or referendum - but was dealt a severe blow in when it was spurned by French and Dutch voters.
The constitution was put on hold, and instead, Lisbon Treaty was agreed after long negotiations, and signed December 2007.
While preserving the substance of the constitution, it is simply amends previous European treaties, rather than marking any fundamental new shift in powers.
The new pact also nearly foundered when it was rejected by voters in Ireland in a 2008 referendum. But EU leaders persevered and the Irish approved it in a second referendum in October 2009, allowing ratification to be completed in the following month.
The Lisbon Treaty created a new post - president of the European Council. Elected for a two-and-a-half-year term, it replaces the practice of rotating the council presidency among member states every six months.
The first holder, Herman van Rompuy, was elected in 2009. He was succeeded by former Polish Prime Minister Donald Tusk in 2014.
The treaty ushered in a wider rebalancing of powers. The European Parliament was put on an equal footing with the Council for most issues, including the crucial areas of the budget and agriculture.
National vetoes were also removed in some areas, although they will remain on tax, foreign policy, defence and social security.
In addition there will be a redistribution of voting weights among member-states, to be phased in between 2014 and 2017, with 55% of member-states being entitled to pass certain measures as they account for 65% of the EU population.
A plan to reduce the size of the European Commission - roughly speaking, the EU's executive - from 2014 so that not every member-state will have its own commissioner was watered down, however.
The current president of the commission is former Luxembourg Prime Minister Jean-Claude Juncker.
He was elected - despite objections from the UK, which considered him too keen a support of a fully federal EU - in October 2014, having been the candidate of the largest party in the European Parliament in the May 2014 European elections.
Among challenges facing him are the continuing instability of the eurozone, and failure to reach agreement on how to deal with waves of refugees and migrants from Syria, who began arriving in the Balkans in large numbers in the summer of 2015.