A lower court made waves in April when it found that Breivik's human rights were violated as he was subjected to "inhumane" and "degrading" treatment in prison -- a decision that disturbed many families of the victims, mostly teenagers at a summer island camp.
"We hope that the state wins this new round, that justice digs deeper into the case," said the head of a family support group, Lisbeth Kristine Royneland, whose 18-year-old daughter was shot dead by Breivik in the killing spree on Utøya island.
Breivik is imprisoned in a 30 square-metre three-cell complex where he's allowed to play video games and watch television on two sets. The 37-year-old also has a computer without internet access, gym machines, books and newspapers.
But beyond these comfortable material conditions, a district court judge had ruled that security measures took excessive precedence over human rights.
She pointed to the fact that Breivik had been kept isolated from other inmates "in a prison inside a prison", without enough social activities.
The ruling also questioned the many potentially "humiliating" strip searches, the systematic use of handcuffs and other frequent awakenings at night, especially in the early days of his imprisonment.
On July 22nd, 2011, Breivik carried out two attacks, first killing eight people by detonating a bomb at the foot of a government building in Oslo.
Then, disguised as a policeman, he killed 69 others by opening fire at a Labour Party youth camp on the Utøya island with the teenagers trapped by the freezing waters of the surrounding lake.
The attacks were the worst committed on Norwegian soil since World War II.
Breivik, a self-proclaimed neo-Nazi who said he killed his victims because they valued multiculturalism, was sentenced in August 2012 to 21 years in prison, a term that can be extended if he is still considered a threat.
Some survivors hailed humanity's victory over an inhumane killer in the April ruling against the Norwegian state, but it pained many relatives of the victims and was criticised in the media.
During the lower court's hearing, Breivik repeatedly provoked onlookers by making a Nazi salute and complaining about cold coffee and frozen meals.
The Norwegian press described the ruling as "wrong" and "difficult to digest". Unsurprisingly, the state appealed.
Attorney General Fredrik Sejersted has insisted that "there is no evidence that Breivik is physically or mentally affected by his prison conditions."
The state maintains that Breivik is not isolated, arguing that he has regular contact with guards and other professionals.
They say his separation from other prisoners is for his own security and that of others.
Breivik's lawyer Øystein Storrvik counters that "the state has not put in place concrete measures to remedy Breivik's mental vulnerability and damage due to prolonged isolation."
Some changes however have been made to his prison conditions in recent months, including replacing a glass wall with a grid during his lawyer's visits and increased contact with guards.
But Storrvik considers these measures to be insufficient and has called for his client to be able to interact with other detainees.
The Oslo Court of Appeals will hear the civil case at Skien prison where Breivik is being held for security reasons. The court will also have to rule on an appeal by Breivik, regarding his inability to freely communicate with the outside world.
On that point, the judge in April ruled in favour of the state, which closely monitors and filters the prisoner's correspondence to prevent him from forming a network capable of carrying out new attacks.
Breivik claims this violates his right to privacy.
The trial is expected to last six days and the date of the appeals court's decision is not yet known.