Tuol Sleng Genocide Museum provides a chilling insight into the ruthless leadership of the Khmer Rouge. The former prison is housed in the building of a high school was used by the Khmer Rouge to interrogate and torture oponents of the regime. The prison was used between 1975 and 1979 to house and torture some 10000 prisoners, many who met their death at the nearby killing fields. The museum has preserved the prison exactly as it was found when the Khmer Rouge were ousted in 1979. You are free to walk around many of the cells and torture rooms which provides a chilling insight into the crimes commited here. Other rooms show pictures of the victims and paintings, by a former prisoner, of the torures occuring. Finally there is and excellent film to watch about the prison. This is a must see in Phnom Penh.