Tuol Sleng is a former Khmer Rouge Prison in Phnom Penh, Cambodia. It was also known as S-21 (Security Office 21). The building was a school before it was converted to a prison in May 1976. Only 7 of the estimated 17,000 victims who were imprisoned and tortured here survived. Imprisonment usually averaged 2 to 4 months. People of many nationalities and backgrounds died here including Vietnamese, Laotians, Thais, Indians, Pakistanis, British, Americans, Canadians, New Zealanders, Australians. The vast majority however were Cambodians.

Prisoners were accused of activities against Angkar, the Khmer Rouge ruling body. Conditions in the prison were brutal. Inmates were shackled at the ankles with iron bars, slept on the floor with few clothes and without blankets and they were required to ask permission to do anything - even to change their sleeping position in the night. Severe beatings were received if any such rules were broken.

Civilian prisoners included farmers, engineers, intellectuals, professors, teachers, students - even Buddhist monks. All were executed, as were their families from the eldest down to new-born babies. Prison guards were both male and female and typically aged between 10 and 15 years. Although they started out as normal individuals they became ever more cruel as time went on.