Tall, blond and with piercing blue eyes, the 33-year-old right-wing extremist has confessed to killing 77 people on July 22nd, 2011, when he gunned down youths attending a Labour party camp after setting off a bomb outside the government offices in Oslo.
The massacre was "a preventive attack against state traitors" guilty of "ethnic cleansing" due to their support for a multicultural society, Breivik told a court hearing in February.
Born on February 13th, 1979 in tranquil and affluent Norway, Breivik grew up without anyone around him suspecting what would one day unfold.
He has said he had an unremarkable childhood, with a diplomat father and a nurse mother who divorced when he was just one year old.
"I have had a privileged upbringing with responsible and intelligent people around me," he wrote in a 1,500-page manifesto he published just before the massacre.
Raised by his mother in a middle-class family, he said he never had financial problems and has only one gripe: "I had way too much freedom though if anything."
But from a young age, child welfare services were concerned that he may not have been receiving proper care.
"Anders has become a contact adverse, somewhat anxious, passive child ... with a feigned, disarming smile," a psychologist wrote when the boy was just four years old.
"Ideally he should be placed with a stable foster family," the expert wrote in a report revealed by Norwegian media.
But that never happened. Around the same time, Anders' father failed in his bid to obtain custody of his son.
After this episode, Anders Behring Breivik appeared to go on to have a typical childhood with no major problems.
"When he was younger, he was an ordinary boy but not very communicative. He was not interested in politics at the time," his father told Norwegian media.
The diplomat cut off all contact with his son when he was around 15, supposedly when Anders, during a hip-hop phase, was caught drawing graffiti tags.
His old friends describe him as a discreet person, who sometimes had a hard time finding his place in the world -- not at all the natural leader he presents himself to be.
He quit high school at age 18 without getting his diploma, supposedly to undertake a career in politics.
In 1999 he joined the populist right-wing, anti-immigration Progress Party and was active with its local youth branch.
He left the party in 2006, writing later on an internet forum that he felt the party was too open to "multicultural demands" and "the suicidal ideas of humanism".
While his criticism of Islam, multiculturalism and Marxism are all over the internet -- where he was very active -- Breivik considered himself to be "a laid back type and quite tolerant on most issues".
"Due to the fact that I have been exposed to decades of multicultural indoctrination I feel a need to emphasise that I am not in fact a racist and never have been," he wrote.
"Being a skinhead was never an option for me. Their dress codes and taste of music was unappealing and I thought they were too extreme," he wrote, adding that he had "dozens of non-Norwegian friends during my younger years".
On his Facebook profile, Breivik describes himself as "conservative", "Christian", and interested in hunting and video games like "World of Warcraft" and "Modern Warfare 2", which, he later revealed, he used to train for his deadly rampage.
On July 22nd on the island of Utøya, the man who normally is disturbingly courteous spent more than an hour methodically killing 69 people, most of them adolescents, in what is believed to be the deadliest shooting ever carried out by a single person.
Just prior, he killed eight others when he blew up a bomb in a van parked in the government block in Oslo.
He called his actions "cruel but necessary", a plan he apparently spent years plotting and then carried out alone.
According to his manifesto, Breivik began his ideological crusade back in 2002 as part of the "Knights Templar" -- an organisation whose existence police have never been able to confirm.
He put his plan into action in late 2009, preparing in minute detail the bloodiest attack on Norwegian soil since World War II and making sure to arouse no suspicions.
He became a textbook example of the "lone wolf" who lived a reclusive life in an apartment with his mother before renting a farm, a move that enabled him to discreetly acquire the fertilisers he needed to build his bomb.
"For me he just looked like your average guy. He could easily go unnoticed," a neighbour told AFP.
"A well-kept Norwegian that no one would suspect," he said.
Such premeditation, such attention to detail, and an ability to avoid detection -- are those the acts of a criminally insane person, as a first court-appointed psychiatric evaluation found him to be?
A second psychiatric report said he was not psychotic at the time of the attacks, but it will ultimately be up to the Oslo district court judges to decide.