Here are key facts about Cambodia's long road from genocide to justice:
-- The communist Khmer Rouge seized control of Cambodia in April 1975 and immediately began dismantling modern society in their drive to transform the country into an agrarian utopia.
The regime abolished religion, schools and currency and exiled millions of people onto vast collective farms. Up to two million people died of starvation and overwork or were executed from 1975 to 1979. The horrors of the genocide were portrayed in the film "The Killing Fields."
WHAT HAPPENED NEXT:
-- The Khmer Rouge were driven from power in 1979 by Vietnamese troops and former regime members who defected, including Hun Sen, now Cambodia's prime minister. He was a mid-level military commander until fleeing to Vietnam in 1977. Under him, the Cambodian government fought the Khmer Rouge until the movement collapsed in the mid-1990s.
-- Cambodia and the United Nations signed an agreement in 2003 which essentially brought the tribunal into being and set out its mandate. Known as the Extraordinary Chambers of the Courts of Cambodia (ECCC), it is a complex hybrid court combining elements of international law with Cambodia's judiciary.
Its mandate is to prosecute "those most responsible" for crimes committed between 1975 and 1979.
The tribunal has faced controversy over allegations that Cambodian staff were forced by their superiors to pay kickbacks for their jobs.
It can impose a sentence of imprisonment of up to life in prison. There is no death penalty and no financial compensation for victims.
It is funded by foreign nations, the biggest donors being Japan and Germany.
WHO FACES TRIAL:
-- After Duch, the court is scheduled to try Khmer Rouge "Brother Number Two" Nuon Chea, former head of state Khieu Samphan, ex-foreign minister Ieng Sary, and his wife, former social affairs minister Ieng Thirith, all on charges of war crimes and crimes against humanity.
The court is also due to rule whether to pursue charges against other former leaders after the Cambodian co-prosecutor said such a move would destabilise the country. Many observers say the decision on whether to prosecute more Khmer Rouge suspects represents a test of the court's independence from the current Cambodian government.
AND THOSE WHO ESCAPED:
-- Because of the tribunal's limited scope, thousands of lower-level Khmer Rouge members and fighters who carried out the regime's brutal acts will never face court. Also escaping justice are "Brother Number One" Pol Pot, who died in 1998, and military commander Ta Mok, one of the regime's most vicious figures, who was in jail when he died in 2006.