Anders Behring Breivik's defence team will call for his acquittal, claiming he acted in self-defence, when his trial opens tomorrow.
Breivik has confessed to killing 77 people in last year's bombing and shooting attacks in Norway.
He is to face charges in the Oslo District Court of terrorism and premeditated murder.
A key issue during the 10-week trial is whether the 33-year-old will be considered sane and accountable for his actions, should he be found guilty.
Two teams of court-appointed psychiatrists who have assessed his mental health are at odds over this.
Just a week before the trial, a new assessment showed Breivik was "not psychotic" at the time of the attacks but noted there was a "high risk that violent crime would be repeated".
In November, two other court-appointed psychiatrists concluded that Breivik was legally insane and suffering from paranoid schizophrenia, suggesting he should be committed to a secure psychiatric unit.
In a recent open letter to newspapers via his attorneys, Breivik has maintained he is sane, stating that being committed to a psychiatric ward would be a fate "worse than death".
The first week of the trial has been reserved for Breivik's testimony. Defence attorney Geir Lippestad has cautioned it will be "very challenging" and that Breivik might even express regret that the death toll was not even higher.
He will claim he "acted in self-defence", Lippestad was quoted as saying by the Dagbladet on Friday.
"Technically, we have no other option but to state his arguments about why he did what he did. We are of course aware that this will not win through, but we are obliged to convey his arguments."
"What we will do is ask questions that will give him an opportunity to explain what he means. Our task as defenders is to call for an acquittal, according to his request, based on his claim of self defence." Breivik has shown no remorse and has said his actions were designed to punish the Government for its pro-immigration policies.
During the trial, the defence plans to call other psychiatric experts.
Also expected is testimony from three Islamists to explain an ideology with which Breivik has said he was at war.
Other witnesses are to include experts on political ideologies and supporters of far-right groups.
According to the indictment, Breivik detonated a self-made 950kg bomb in a government district in Oslo that killed eight people.
Later, 69 people died at a Labour Party summer camp on an island near the capital, after Breivik allegedly embarked on a shooting spree.
Sixty-seven people died of gunshot wounds and two drowned or fell to their deaths while trying to flee.
Of the 69 who died on the island of Utoya, 34 were aged 14 to 17 and 22 were aged 18 to 20. In addition, 33 suffered non-fatal gunshot wounds.
Survivors of the attacks are due to testify, as are medical examiners who performed the autopsies.
The summer camp was attended by youth from all over Norway.
"Giving testimony will be a way of moving on," Tarjei Jensen Buch told news agency NTB in Tromso, northern Norway.
The 20-year-old Buch - who is undergoing rehabilitation after sustaining a gunshot wound to the leg and other injuries - said he planned to travel abroad during part of the trial before testifying next month.
He anticipated it would be "extremely tough" to face Breivik again.
The public response to the attacks included a "rose march" attended by tens of thousands of people in Oslo days after the killings, in a manifestation against violence and to honour the victims.
Authorities have also issued several reports questioning whether the official response to the attacks could have been faster.