The Tuol Sleng Genocide Museum is a museum in Phnom Penh, the capital of Cambodia. The site is a former high school which was used as the notorious Security Prison 21 (S-21) by the Khmer Rouge regime from its rise to power in 1975 to its fall in 1979. Tuol Sleng in Khmer;  means "Hill of the Poisonous Trees" or "Strychnine Hill".

Formerly the Chao Ponhea Yat High School  named after a Royal ancestor of King Norodom Sihanouk, the five buildings of the complex were converted in August 1975, four months after the Khmer Rouge won the civil war, into a prison and interrogation center. The Khmer Rouge renamed the complex "Security Prison 21" (S-21) and construction began to adapt the prison to the inmates: the buildings were enclosed in electrified barbed wire, the classrooms converted into tiny prison and torture chambers, and all windows were covered with iron bars and barbed wire to prevent escapes... More