Mahmoud Zahhar is believed to be the leader of the Palestinian Islamic Resistance Group, Hamas, in Gaza.
Mahmoud Zahhar insists Hamas has a "right to resist"
A surgeon who teaches medicine at the Islamic University in Gaza, Mr Zahhar helped found the group in 1987 with Sheikh Ahmed Yassin.
He became a member of the "collective leadership" of the militant group in 2004 after Sheikh Yassin and Abdel-Aziz Rantissi were assassinated by Israel.
Mr Zahhar is one of Hamas' ideological leaders and is considered to be more hardline than Ismail Haniya, who headed the group's national list of candidates for the January legislative elections.
Mr Zahhar was born in 1945 in the Zeitoun neighbourhood of Gaza City to a Palestinian father and an Egyptian mother.
Having spent much of his youth in Egypt, he remained in the country to study medicine at Cairo's Ain Shams University.
After graduating in 1971, Mr Zahhar spent a further five years at Ain Shams specialising in general surgery.
He then returned to the occupied territories to lecture at the Department of Medicine of the newly created Islamic University of Gaza, where Rantissi also worked.
There, Mr Zahhar joined the local offshoot of Egypt's oldest and largest Islamist organisation, the Muslim Brotherhood.
In 1987, a mass uprising - or intifada - against the Israeli occupation began in Gaza and quickly spread to the West Bank.
Mr Zahhar was Sheikh Ahmad Yassin's personal physician
Palestinian Muslim Brotherhood leaders in Gaza became increasingly worried that they were losing support to more radical and militant Islamist groups, such as Islamic Jihad, and decided to form Hamas.
The group's covenant, published in August 1988, rejected any compromise with Israel and called for an Islamic state to be founded in its place. The Israeli authorities banned Hamas shortly afterwards.
Mr Zahhar and Rantissi, went on to lead the group after Sheikh Yassin was arrested by Israel in 1989 and sentenced to life imprisonment for ordering the killing of Palestinians who had allegedly collaborated with the Israeli army.
Mr Zahhar also became Hamas' unofficial representative to Yasser Arafat's Palestinian Liberation Organisation (PLO) in 1990.
In December 1992, Mr Zahhar, his brother, Fadel, and Rantissi were among more than 400 Islamic activists deported by Israeli Prime Minister Yitzhak Rabin to South Lebanon.
Rantissi and Zahhar were deported to Lebanon in 1992
The activists spent more than a year camped at Marj al-Zahour, where Hamas received unprecedented media exposure and became known throughout the world.
Though Mr Zahhar and Rantissi were allowed to return to Gaza a year later, 18 activists, including his brother, remained in Lebanon.
Back in Gaza, Mr Zahhar soon clashed with the Palestinian authorities. He was arrested several times by Palestinian security forces and at one stage spent seven months in a Palestinian jail.
With the outbreak of a new intifada in September 2000, Hamas's popularity among the Palestinian population grew rapidly as the organisation's military wing, the Izzedine al-Qassam Brigades, killed a large number of Israeli civilians in a new campaign of suicide bombings.
Israel responded by targeting Hamas' military and political leaders.
Israel has vowed to eliminate Hamas' entire leadership
On 10 September 2003, only three days after a failed attempt to kill Sheikh Yassin, Israel bombed Mr Zahhar's home in Gaza, destroying it completely.
Mr Zahhar survived the attack, but his 25-year-old son, Khaled, and a bodyguard were killed.
Sheikh Yassin and Rantissi were not, however, so fortunate.
An Israeli air strike killed Hamas' spiritual leader on 22 March 2004, and Rantissi was assassinated just weeks after it was announced that he would take over as leader in the Gaza Strip.
Wary of further attacks, Hamas kept the appointment of Rantissi's successor secret.
However, Palestinian sources said the group's new leaders in Gaza were Mr Zahhar, Ismail Haniya and Said al-Siyam.
'Right to resist'
Since then, Mr Zahhar has been influential in getting Hamas involved in the Palestinian political process.
Hamas has kept the identities of its leaders in Gaza secret
The group agreed to an informal truce with Israel which began in February 2005, and Hamas took part in elections for the first time.
Although the international community has called on Hamas to renounce violence, Mr Zahhar has insisted his organisation has the "right to resist" Israeli attacks.
"We are not playing at terrorism or violence. We are under occupation," he said.
"The Israelis are continuing their aggression against our people, killing, detention, demolition and in order to stop these processes, we run effective self defence by all means, including using guns."
While insisting on the right to "resist" he has also floated the possibility of holding peace talks with Israel via a third party.