He was convicted of terrorism in 2012 and sentenced to 21 years in prison - Norway's maximum sentence, which can be extended when criminals are deemed a threat.
Nearly five years later, Breivik, 37, has made his way back to the courtroom this week. This time, he is suing authorities for perceived inhumane treatment behind bars.
Human rights experts say his case reignites a debate about the justice system - highlighting the balance between punishment and the right to fair and humane treatment.
On July 22, 2011, Breivik, a far-right extremist, drove a white Volkswagen van into Oslo's government district and ignited a bomb, according to GQ.
There, authorities said, he killed eight people.
He then drove west to an island, Utoya, and opened fire - mainly on teenagers - at a summer camp for the youth league of the left-leaning Labor Party.
"I'm going to kill you all," Breivik purportedly told them, according to a survivor's account in GQ. "You're all going to die."
Sixty-nine more were killed.
Earlier this week, Breivik was escorted into a makeshift courtroom inside a gymnasium in Norway's Skien prison to call out authorities for violating the European Convention on Human Rights. Moments later, he gave them the Nazi salute.
On Wednesday, he vowed to the court that he would fight for National Socialism until the very end.
"I have been a dedicated National Socialist since I was 12," he said at trial, according to Reuters.
Breivik also compared himself to Nelson Mandela, the South African anti-apartheid leader, according to CNN, saying the difference between the two is Mandela "ordered action" and Breivik "carry out the action."
Through his lawsuit, Breivik is arguing that those responsible for his care have violated the European Convention on Human Rights, which protects a person's right "to respect for his private and family life, his home and his correspondence" and prohibits "inhuman or degrading treatment or punishment."
I have been a dedicated National Socialist since I was 12
During his testimony Wednesday, he complained that he had been strip-searched more than 800 times and that the microwaved meals they feed him are "worse than waterboarding," according to reports.
"For the past five years the state has tried to kill me," he said, according to BBC News. "I don't think many people would have survived as long as I have."
Norway, which has abolished the death penalty as well as life sentences, is known for its humane approach to incarceration.
Breivik has been in isolation in Skien, not far from Oslo, in a three-room cell - with separate areas for sleeping, studying and exercising.
In his suite, he can type notes on a laptop (without Internet), run on a treadmill, watch TV and DVDs or play games on a Sony PlayStation, according to the New York Times. He has also been permitted to take correspondence courses at the main university.
But Breivik has been unhappy with his stay.
In 2012, Breivik penned a 27-page letter to officials, listing his complaints, the New York Times reported at the time, citing the Norwegian newspaper Verdens Gang.
He didn't have a thermos to keep coffee from turning cold.
He wasn't allowed to keep moisturizing lotion for his skin.
The switches for his TV and his light were outside of his cell.
Then there was his writing pen - designed to keep inmates from stabbing themselves or someone else. He called it a "nightmare of a tool" that made his hand hurt.
More seriously, Breivik has complained about strip-searches, censorship and isolation - which his attorneys have said "isn't human," according to the New York Times.
Indeed, since Breivik was incarcerated, he has been separated from other inmates, and his interaction with professionals has been from behind a glass screen. His mother was the only person permitted to meet with him face-to-face, and she died in 2013, according to the BBC.
Years ago, Breivik wrote a 1,500-page anti-Islam and anti-liberal manifesto called "2083: A European Declaration of Independence," in which he noted that prisons were an ideal place to recruit followers.
That's why, authorities told the Guardian, they have been limiting Breivik's contact with other inmates and closely monitoring his phone calls and letters to keep him from establishing an "extremist network." Government attorneys said some 600 letters have been withheld from about 4,000 letters that were either written by Breivik or to him, because of those security concern, according to the Associated Press.
David Fathi, director of the American Civil Liberties Union's National Prison Project, said that balance between punishment and protecting an inmate's human rights is a "difficult balance to strike."
"We want to hold people accountable for very serious crimes of this nature and we want to keep them from doing additional harm but don't want to torture them and we don't want to damage them to the point where they can't live in society," Fathi told The Washington Post. "So that is the challenge of a humane and progressive criminal justice system."
Fathi said that regardless of how comfortable Breivik's physical surroundings are, "deprivation of human contact" can cause excruciating pain.
"Prisoners who have suffered the most extreme forms of abuse and mistreatment, from John McCain to Nelson Mandela, say that solitary confinement is the worst of all," he said, adding that Breivik's complaint about isolation "is not a frivolous argument."
Norway's parliamentary ombudsman, who looked into Breivik's claims, released a report last year saying that Breivik's isolation could turn into "inhumane treatment," the Guardian reported at the time.
"The regimen in the very high security unit imposes very strict conditions on inmates' freedom of movement and their possibility to have contact with other people," ombudsman Aage Thor Falkanger, who investigates such claims, wrote after he visited Breivik's prison conditions, according to the newspaper.
He added: "This, and the fact that in reality there is an extremely limited number of inmates in the very high security unit, means that this regimen represents an elevated risk of inhumane treatment."
Falkanger recommended more interaction between guards and inmates to "reduce the risk of damage" from isolation and using "less intrusive security measures than handcuffs," the Guardian reported.
The court hearings in the case were set to begin this month.
The goal, Breivik's attorneys said, is for Breivik to gain contact with other inmates and face fewer restrictions on his communication with the outside world, according to the Associated Press.
"There's justification for physical separation," said Fathi, with the ACLU, "but you can have physical separation without social isolation. That's the challenge in managing prisoners who are dangerous or who may be in danger themselves."
