The former head of a notorious Khmer Rouge prison is due to appear in front of the UN-backed Cambodia genocide tribunal at its first public session.
Kang Kek Ieu, also known as Comrade Duch, ran Tuol Sleng jail in Phnom Penh where around 14,000 people were tortured, with most of those dying.
More than a million people are thought to have died during the four years of Khmer Rouge rule between 1975 and 1979.
Duch was the first of five surviving leaders to be targeted by the court.
On Monday, the former Khmer Rouge head of state, Khieu Samphan, was formally charged with committing crimes against humanity.
The 76-year-old, a former close confidant of the movement's leader Pol Pot, was arrested at a hospital in Phnom Penh and taken to appear before the tribunal.
The BBC's Guy De Launey in Phnom Penh says the appearance of Duch, the youngest surviving member of the Khmer Rouge's leadership, will be a real milestone for the special tribunal.
A protracted civil war and political and legal wrangling caused decades of delay in setting it up, so the appearance of a suspect in open court is a significant event, our correspondent says.
"This will be a first full-blown hearing for the people to be able to see that justice is being done," the co-prosecutor, Robert Petit, told the BBC .
"I hope that it will provide Cambodians with a certain sense of relief that the process is ongoing and is transparent, or as transparent as it can be."
Our correspondent says there are unlikely to be any explanations from the 65-year-old, who once ran a prison later labelled a "killing machine", at the hearing on Tuesday.
Instead Duch is likely to seek bail after spending more than eight years in jail without trial, he adds.
The hearing is expected to take place over two days, with the judges announcing their decision early in December.
Duch is unlikely to face trial until the middle of next year.