He says that he has information about Al-Qaeda operations but has not so far revealed anything.
He is charged with taking part in planning for the 11 September attacks -- the only person charged in connection with the attacks.
Twentieth hijacker?
He is believed by many law enforcement officers to be the "twentieth hijacker" who was prevented from carrying out his mission only because he was already under arrest.
However some reports recently have suggested that he might instead have been planning another, later, attack.
Moussaoui was detained three weeks before 11 September on immigration charges after a flying school in Minnesota reported that he had been acting suspiciously.
According to the FBI, he had been on a jumbo jet simulator but had shown no interest in takeoffs or landings -- only in how to control the plane in the air.
Zacarias Moussaoui is a French citizen of Moroccan descent. He was born in 1968 in south-west France and came to Britain in the early 1990s to enrol as a student at London's South Bank University from which he graduated in 1997.
He lived in south London on and off for nine years.
His brother, Abd-Samad Moussouai was quoted by The Independent newspaper as saying that Moussouai became involved in Islamic extremism while living in London.
French interest
Moussaoui is suspected of having links with Sheikh Abu Qatada, the London-based Islamic cleric named in October in a Treasury list of terrorist suspects. Sheikh Qatada has rejected any links with terrorism.
French Intelligence was interested in him and warned the Americans but he was always one jump ahead and nobody "joined up the dots" about him, even after he was detained.
According to the indictment against him, Moussaoui went for training at an Al-Qaeda camp in Afghanistan in 1998.
He was in Pakistan at the end of 2000 before going to London and then onto the United States in February 2001.
There, he enrolled in one flying school in Oklahoma and then paid $6000 cash to join the school in Minnesota.
After his arrest, he said nothing.
Moussaoui's mother, who lives in France, believes he is innocent of any involvement in the 11 September attacks.
She said: "They took two months [to announce charges] - if they'd had anything real against him, they would have done it right away, no?"