The number of MEPs will increase to 754 if the Lisbon Treaty comes into force.
The political groups in the European Parliament are mostly broad coalitions. Most MEPs sit in three blocs - centre-right, centre-left and liberal. But there are also smaller groups, such as greens, anti-federalists and the hard left.
Now there are 736 MEPs, compared with 785 in the 2004-2009 parliament.
GUE/NGL (left-wing) - 35 seats
This parliamentary group is a union of two smaller organisations, the European United Left and the Nordic Green Left.
Some members belong to their national communist parties.
The bloc says it is "firmly committed to European integration, although in a different form from the existing model".
It emphasises the need to tackle unemployment, ensure social justice and develop solidarity among EU member states.
It calls for a Europe "without the democratic deficit" and "free from neo-liberal monetarist policies".
S&D - Progressive Alliance of Socialists and Democrats in Europe (centre-left) - 184 seats
This is the group for Europe's Socialist and Social Democrat parties, including the British Labour Party. The biggest national delegations are from France, Spain and Germany.
The political groups, explained with cake
It is united in supporting the Lisbon Treaty.
The S&D describes its priorities as: "safeguarding employment and living standards against the recession, tackling climate change, promoting social justice, security and fairness".
It wants a European Pact for decent minimum wages across the EU.
Many S&D members accuse European conservatives of contributing to the recession by placing too much faith in the free market and allowing light-touch regulation.
Most MEPs in the S&D formerly belonged to the Party of European Socialists (PES). The bloc changed its name after the June 2009 election because it took in some MEPs from the Italian Democratic Party, who previously sat with the liberal bloc.
The PES grouped 216 MEPs, but the election delivered setbacks for social democrats in many EU countries, and the S&D has 184 members - albeit in a smaller parliament.
ALDE - Alliance of Liberals and Democrats for Europe (liberal) - 84 seats
The ALDE favours further deepening and enlargement of the EU, and what its leader, Graham Watson, calls "finding supra-national answers" to pan-European challenges.
The liberals want the European single market to function better, with more freedom of movement for workers and more competition in areas such as energy, postal services and financial services.
The main pan-European party in the ALDE is the European Liberal, Democrat and Reform Party (ELDR). Its manifesto puts the defence of civil liberties top of the agenda.
After the election, Ireland's governing Fianna Fail party moved from the nationalist UEN group - now defunct - to the ALDE. The party will have ALDE support for its "Yes" campaign ahead of a fresh Irish referendum on the Lisbon Treaty in early October.
EPP - European People's Party (centre-right) - 265 seats
This centre-right group is dominated by Europe's Christian Democratic parties.
It remains the biggest grouping in the parliament, despite the departure of its more Eurosceptic MEPs, headed by Britain's 25 Conservatives.
The EPP wants closer economic integration in Europe, as well as common immigration, defence and foreign policies. It was opposed to the UK having a referendum on the Lisbon Treaty - something the Conservatives campaigned for.
Its programme calls for protection of family values, far-reaching EU budget reform and a firm transatlantic partnership.
The biggest national delegations in the EPP are German, Italian and Spanish. The EPP gained some Italian politicians who previously sat with the conservative UEN bloc. The election produced especially strong results for centre-right parties in power in France, Germany, Italy and Poland.
ECR - European Conservatives and Reformists Group (right-wing) - 54 seats
This new group's members are stalwart believers in national sovereignty and opponents of European integration.
The biggest delegations come from the UK (the Conservatives), Poland (the conservative Law and Justice Party) and the Czech Republic (the Civic Democratic Party - ODS).
David Cameron's Conservative MEPs led the charge to establish the new group - yet it is now headed by a controversial Polish MEP, Michal Kaminski, whose appointment was marred by a bitter row. In the past he has used derogatory language when referring to gay people, though now he insists he is not homophobic.
A veteran British Conservative MEP, Edward McMillan-Scott, defied his party and got elected as one of the parliament's new vice presidents - instead of Mr Kaminski, who was the ECR's candidate. As a result, Mr McMillan-Scott - who maintains close ties to the EPP - was expelled from the British Conservative group.
Non-attached MEPs include French National Front veteran Jean-Marie Le Pen, who has been an MEP since 1984, and his daughter Marine Le Pen, vice-president of the National Front.
Parties new to Strasbourg include the far-right British National Party, with two MEPs, the anti-immigration Freedom Party (PVV) from the Netherlands and the Swedish Pirate Party, which campaigns for freedom on the internet.
The ranks of MEPs belonging to no group swelled when the far-right Identity, Tradition and Sovereignty (ITS) bloc disbanded in November 2007, after a row between its Italian and Romanian members over race.
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