Is the site of a former orchard and mass grave of victims of the Khmer Rouge – killed between 1975 and 1979 – in Dangkao Section, Phnom Penh, Cambodia, about 17 kilometers (11 mi) south of the Phnom Penh city center. It is the best-known of the sites known as the Killing Fields, where the Khmer Rouge regime executed over one million people between 1975 and 1979. Mass graves containing 8,895 bodies were discovered at Choeung Ek after the fall of the Khmer Rouge regime. Many of the dead were former political prisoners who were kept by the Khmer Rouge in their Tuol Sleng detention center and in other Cambodian detention centers. Today, Choeung Ek is a memorial, marked by a Buddhist stupa. The stupa has acrylic glass sides and is filled with more than 9,000 human skulls. Some of the lower levels are opened during the day so that the skulls can be seen directly. Many have been shattered or smashed in. Tourists are encouraged by the Cambodian government to visit Choeung Ek.

The Tuol Sleng Genocide Museum or simply Tuol Sleng is a museum chronicling the Cambodian genocide. Located in Phnom Penh, the site is a former secondary school which was used as Security Prison 21 by the Khmer Rouge regime from 1975 until its fall in 1979. To accommodate the victims of purges that were important enough for the attention of the Khmer Rouge, a new detention center was planned in the building that was formerly known as the Tuol Svay Pray High School, named after a royal ancestor of King Norodom Sihanouk, the five buildings of the complex were converted in March or April 1976 into a prison and an interrogation center.