The verdict will bring a close to one of the most spectacular trials in Norway's history.
The rightwing extremist has confessed to the attacks so there is no doubt about his guilt, but the question of his sanity was the focus of his 10-week trial that wrapped up in June.
On Friday, the five judges at the Oslo district court will announce whether they consider the 33-year-old legally responsible for his crimes, which determines whether he will spend a long sentence behind bars or in a closed psychiatric ward.
On July 22 2011, Breivik set off a car bomb outside the government offices in Oslo, killing eight people, before going to the island of Utoeya, northwest of the capital, where he spent more than an hour gunning down another 69 people, mostly teenagers, attending a Labour Party youth camp.
The attacks traumatised the normally-tranquil nation, and highlighted authorities' lack of preparedness.
Norway's national police commissioner resigned last week after a scathing report on the response to the attacks, and Prime Minister Jens Stoltenberg is due to appear before an extraordinary parliamentary session to discuss the report at the end of the month.
Charged with "acts of terror", Breivik could possibly be sent to closed psychiatric care for life. Or he could be sentenced to up to 21 years in prison, the maximum sentence in Norway though it could be extended indefinitely as long as he is considered a danger to society.
'Cruel but necessary'
While Breivik has confessed to the crimes he pleaded not guilty, arguing that his actions were "cruel but necessary" to protect his country from multiculturalism, which was embraced by his victims.
In a rare reversal of roles, the prosecution has called for Breivik to be ordered to undergo psychiatric care, while the defence has asked that he be found sane even though that means he faces a long prison sentence.
Prosecutor Svein Holden explained in his closing arguments that he would rather err on the side of caution, stressing that "it would be worse to sentence someone who is psychotic to prison than to send someone who is not psychotic to psychiatric care."
His position is based on a court-ordered psychiatric evaluation that found the killer to be suffering from paranoid schizophrenia.
While a second opinion also ordered by the court later found no sign of psychosis, the prosecution team said the first assessment raised enough doubt that they wanted Breivik to undergo care.
Meanwhile, defence lawyer Geir Lippestad called for a prison sentence, at his client's request.
"It is just as bad to treat a healthy individual [in a psychiatric ward] as to not treat someone who is ill."
Breivik wants to be found sane so that his Islamophobic ideology is not considered the ravings of a lunatic, and has said psychiatric care would be a fate "worse than death".
He considered the conclusions of the first psychiatric evaluation "the ultimate humiliation".
On the final day of his lengthy trial, during which he showed almost no emotion and no remorse – "I would do it again," he said – Breivik asked to be acquitted.
"I was acting on behalf of my people, my religion and my country," he argued.
Families of the victims and survivors remain divided on the question of his sanity, but all are united in their belief that whatever the court decides, Breivik will spend the rest of his days locked up.
"We have faith in the justice system that it will make the right decision based on the facts presented during the trial and within the existing legal framework," Christin Bjelland, the vice president of a victims' support group, said.
"Either way this man will never get out, whether he is found criminally responsible or not," she said.
For security reasons, even if he is ordered to undergo psychiatric care, Breivik will likely spend his days in a high-security prison near Oslo where a wing has recently been turned into a small hospital. – Sapa-AFP