Prior to the Khmer Rouge take-over in 1975, Toul Sleng was the Toul Svay Prey High School. In 1975, when the Khmer Rouge became to power, it was converted by Pol Pot's security forces into the S-21 prison and interrogation facility and soon became the largest center of incarceration and torture in the country. Inmates were systematically tortures, sometimes over a period of months, to extract confessions after which they were executed at the killing Fields. S-21 processed over 17.000 people (including a handful of westerners), less than a dozen of whom survived.

The building now serves as a museum, a memorial and a testament to the madness of the Khmer Rouge regime. Much has been left in the state it was when the Khmer Rouge abandoned it in 1979. Incredibly, the regime kept accounts of all interrogations and confessions which have sice been subject of detailed, historical research and have provided a unique perspective on the paranoia and mistrust that affected the Khmer Rouge regime from top to bottom.

The museum also displays thousands of photos of the victims and an exhibition of paintings of torture at the prison by Vann Nath, a survivor of Toul Sleng.