While there is no doubt the 33-year-old right-wing extremist is the killer -- he has confessed but refused to plead guilty -- the main unresolved question is his mental state and whether he will be sent to prison or a closed psychiatric ward.
On July 22nd, Breivik first set off a car bomb outside government buildings in Oslo, killing eight people and injuring more than 200 others.
He then travelled to the small island of Utøya northwest of the capital where the ruling Labour Party's youth organisation was hosting a summer camp.
Dressed as a police officer, he spent more than an hour methodically shooting and killing another 69 people, mainly terrified teenagers trapped in by the icy waters of the surrounding lake.
Never before has a shooting by a single individual claimed as many victims, according to Jack Levin and James Alan Fox, the authors of several books on serial killers and mass murderers.
"There have been larger massacres using other kinds of weapons, but none so large by gunfire," said Levin, a professor of criminology at Northeastern University in Boston.
"Terrorists are usually interested in maximizing body counts, so they use explosives," as in the Oklahoma City bombing which killed 168 people in April 1995, he wrote in an email to AFP.
Breivik, who has claimed to be on a crusade against the "Muslim invasion" of Europe and the multi-culturalism embraced by Norway's centre-left government and especially Prime Minister Jens Stoltenberg's Labour Party, has described his actions as "cruel but necessary".
In a rare reversal of habitual roles, the defence attorneys are, upon request from their client, arguing that he is of sound mind and therefore responsible for his actions, while the prosecution has said it wants him declared criminally insane, in line with an expert evaluation.
The right-wing extremist, who has said being sent to a psychiatric ward would be "worse than death", wants to be declared sane, according to his lawyers, so as not to damage the political message presented in his 1,500-page manifesto published online shortly before the attacks.
The defence also argues that Breivik should not be locked up forever.
"A life sentence does not exist in Norway. At one point, he will be back out in society, not in the near future, but in many years," his main lawyer, Geir Lippestad, said recently.
But even though Norway has a maximum limit of 21 years behind bars, Breivik could still face life in prison due to a special provision that allows for extensions of his term for as long as he is considered a danger to society.
If the prosecution gets its way and Breivik is found criminally insane, however, he will instead be sentenced to treatment in a locked psychiatric institution, possibly for life.
Late last year, two psychiatric experts carried out a court-mandated evaluation and concluded he was suffering from paranoid schizophrenia and could therefore not be sentenced to prison.
That conclusion caused outcry in Norway, and the Oslo court has ordered a second evaluation by two new experts set to present their findings on Tuesday.
In the end, however, the five Oslo court judges will determine whether Breivik should be considered sane when they present their verdict, probably around mid-July.
Whether he goes to prison or a psychiatric institution, prosecutors say he should never be set free.
"We would have a very hard time seeing him walking the street a free man in a few years," Svein Holden, one of two prosecutors in charge of the case, told AFP.
Ordinarily serene Norway was deeply shocked by the attacks, which unleashed emotional scenes of unity and sparked deep, nationwide self-reflection on the delicate balance between democratic openness and security.
Norway's response to the violence, Stoltenberg vowed after the attacks, would be "more democracy, more openness, more humanity, but without naivety".
Hundreds of journalists from some 210 news organisations from around the world have signed up to cover the 10-week trial, with proceedings in the Oslo district court set to be broadcast live to 17 local courthouses around the country to accommodate more than 770 survivors and families of victims figuring as plaintiffs.
"From the point of view of both the seriousness of the crime and the logistics, this is the most important trial we've ever had to organise," Oslo district court presiding judge Geir Engebretsen said.
Nearly nine months after the carnage, the victims' families meanwhile say they are only waiting for one thing: for justice to be served.
"We want a clean, serious and dignified trial to ensure that the guilty party is convicted and that light is shed on what happened on July 22nd," Trond Blattmann, who heads a support group for the families and who himself lost a son on Utøya, told AFP.