Anders Behring Breivik, who set off a bomb and massacred young people at a camp in Norway, is to go on trial in the capital Oslo.
He is accused of killing 77 people - eight in the car bomb attack in Oslo and the rest - mainly teenagers - at the camp, organised by the governing Labour party, on Utoeya Island.
He has confessed to the killings, but denies criminal responsibility.
He says the party is a "legitimate target" because it tolerates Muslims.
If the court declares Breivik criminally insane, he will be committed to psychiatric care, but if he is judged to be mentally stable, he will go to prison.
He faces a sentence of 12 years, which could be extended to keep him behind bars for the rest of his life.
The BBC's Steve Rosenberg in Oslo says that with Breivik not expected to express any remorse for his actions, his trial promises to be an ordeal for the families of his victims and for those who survived the attacks.
The trial is due to start at 09:00 local time (07:00 GMT).
The first day is expected to be taken up mainly by going through the charges, Breivik's plea and the technicalities of the trial.
The accused is expected to plead not guilty.
Jorid Nordmelan, a survivor of the Utoeya massacre, told the BBC she would be in court to hear Breivik testify.
"It's a historical date for Norwegians," she said. "We never had a trial like this, so we don't know what's going to happen.
"Prosecutors told me they were going to make the opening statements awful, so that people can just feel what he did right there."