John Demjanjuk, an 89-year-old former Nazi camp guard, is due to stand trial on Monday on charges of helping to force 27,900 Jews into gas chambers in 1943.
Demjanjuk is expected to appear in a wheelchair before a packed court in the southern city of Munich at what is likely to be Germany's last major trial from the Nazi era.
German state prosecutors believe Demjanjuk, who was top of the Simon Wiesenthal Center's list of most-wanted war criminals, assisted in murders at the Sobibor death camp, in what is now Poland, where at least 250,000 Jews were murdered.
Jewish groups and victims' families say it is never too late for justice to be done and that the case is symbolic.
"We should not make the mistake of thinking that a case against one war criminal is a case against just one man," said Rabbi Marvin Hier, Dean of the Wiesenthal Center in Los Angeles.
"When the bell tolls for John Demjanjuk, it is also tolling for every other war criminal. Even if it just gives them sleepless nights," he told Reuters.
Demjanjuk, who was born in Ukraine and fought in the Red Army before being captured by the Nazis and recruited as a camp guard, was extradited in May from the United States.
He emigrated to the United States in 1951, becoming a naturalized citizen in 1958, and worked in the auto industry.
He denies he was involved in the Holocaust and his family insists he is too frail to stand trail.
Demjanjuk's son said his father had been in hospital for five days in the last week to undergo a range of tests and had a blood transfusion due to a bone marrow disease.
"They are forcing the trial to go forward regardless of my father's condition," John Demjanjuk Jr told Reuters in a statement.
Life behind bars?
Due to his weak condition, the hearings will be restricted to two 90-minute sessions per day. His lawyer, Guenther Maull, said Demjanjuk was in constant pain and suffered from periods of mental absence.
The trial, expected to last until May, is due to start at 0900 GMT and sessions have been scheduled for three days from Monday for this week. Over 200 journalists have been accredited.
If all goes to plan, the prosecution will read the charges on Monday and Demjanjuk, who could be sentenced to spend the rest of his life behind bars, will have the chance to respond. Prosecutors plan to show the court documents, including an identity card, to prove he was at Sobibor and they will call about 20 witnesses.
While the case has attracted enormous global interest, many Germans would prefer to draw a line under the Nazi past and focus on a Germany's new-found role on the world stage.
Although he has acknowledged being at other camps, Demjanjuk has denied he was in Sobibor, which prosecutors say was run by 20-30 Nazi SS members and up to 150 former Soviet war prisoners.
In the Sobibor gas chambers, Jews died in 20 to 30 minutes after inhaling a toxic mix of carbon monoxide and carbon dioxide, say prosecutors, who argue that Demjanjuk was at the camp for about six months in 1943.