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.
ALLIANCE OF LIBERALS AND DEMOCRATS FOR EUROPE (ALDE)
Members in outgoing (785-seat) parliament: 100
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 will move from the nationalist UEN group to the ALDE. The party will have ALDE support for its "Yes" campaign ahead of fresh Irish referendum on the Lisbon Treaty, expected in October.
EUROPEAN PEOPLE'S PARTY - EUROPEAN DEMOCRATS (EPP-ED)
Members in outgoing (785-seat) parliament: 288
The group is dominated by Europe's Christian Democratic parties - the leading force of the EPP - but also includes the more eurosceptic European Democrats (14% of the total) headed by Britain's Tories.
However, Conservative leader David Cameron plans to take his MEPs out of the EPP-ED after the election.
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 four biggest national delegations in the EPP-ED are German, British, Spanish and Italian. While the bloc may lose British MEPs after the election it may gain more Italians.
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.
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.
To form a political group in the parliament - and to claim the funding and committee posts that go with that status - it is necessary to have 25 members from at least seven of the 27 member states.
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