Zubaydah is accused of building up al-Qaeda in Pakistan
His central position makes him a potential mine of vital information about the names and locations of al-Qaeda members as well as any future attacks.
He ranked third on the US list of al-Qaeda suspects they wished to capture, after Osama Bin Laden and his deputy, Ayman al-Zawahri, but was not on the FBI most wanted list. No charges have been brought against him in the US.
Jordanian links
The 30-year-old Palestinian, born in Saudi Arabia, is also known as Zayn al-Abidin Muhammad Husain and Abd Al-Hadi Al-Wahab.
He has strong connections with Jordanian and Palestinian groups and was sentenced to death in absentia by a Jordanian court for his role in a thwarted plot to bomb hotels there during millennium celebrations.
During the 1990s he was al-Qaeda's chief recruiter, selecting new members and arranging their visits to training camps.
"He receives young men from all countries. He accepts you or rejects you. And he takes care of the expenses of the camps," said Ahmed Ressam, an Algerian witness, during the trial of a man accused of the "millennium plot" at Los Angeles International airport.
US officials allege that Abu Zubaydah was behind that attack.
Master of disguise
Since the death of the al-Qaeda commander of military operations, Muhammad Atef, in a bomb attack in Afghanistan, he moved up the organisation's hierarchy.
He is alleged to have been heading the reorganisation of al-Qaeda in Pakistan, as well as planning new attacks on US interests.
He has alleged connections to a plan to blow up the US Embassy in Sarajevo, as well as a planned attack on the American embassy in Paris.
He is also thought to have briefed Richard Reid, the so-called shoe bomber arrested on board a flight from Paris to Miami in December last year.
Abu Zubaydah was an elusive suspect. Few photographs of him were in existence, he has used at least 37 aliases and is considered a master of disguise.
Officials say he is believed to have recently changed his hairstyle and gained weight to avoid identification.