Hezbollah - or the Party of God - is a powerful political and military organisation of Shia Muslims in Lebanon.
Who are they?
The group was formed - with financial backing from Iran - in response to the Israeli invasion of Lebanon in 1982.
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It has become the most powerful military force in Lebanon, but also has a parliamentary party with MPs, and has held seats in the cabinet.
The group's military wing, the Islamic Resistance, is believed to have 500-600 full time, highly trained and motivated fighters.
Some estimates put the number as high as 1,200, with the ability to call on several thousand less experienced "reserves".
During the 2006 conflict in Lebanon, Hezbollah showed that they are armed with rockets that can reach deep into northern Israel.
What do they want?
Hezbollah was formed primarily to offer military resistance to the Israeli occupation.
Hezbollah has significant popular support in Lebanon
Initially, it proposed an Iranian-style Islamic state, although this was later abandoned in favour of a more inclusive approach.
The group calls for the destruction of the state of Israel. It regards the whole of Palestine as occupied Muslim land and it argues that Israel has no right to exist.
It also demands the release of prisoners from Lebanon who are being held in Israeli jails.
Who supports them?
The Israeli withdrawal from southern Lebanon in May 2000 won Hezbollah the respect of many Lebanese.
It has built broad support by providing social services and health care. The group also has an influential TV station, al-Manar.
Hezbollah is believed to receive military training, weapons and explosives, as well as diplomatic and financial support, from Iran. Syria provides diplomatic, political, and logistical support.
The US and Israel view Hezbollah as a terrorist group. Its members were involved in kidnapping westerners and in attacks on foreign troops who were based in Lebanon during the civil war.
The UK says the military wing of Hezbollah is a terrorist group, but not the political side of the organisation.
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UN Security Council resolutions call for armed militia groups like Hezbollah to disarm.