The Khmer rouge regime was one of the most murderous the world has ever known in the 20th century. In order to understand this phenomenon, one has to study the prison system, in the broader sense of the word, and its place in the repression commanded by this regime. The mass executions were not summary executions in remote places, as it was first thought, but, actually, the endpoint of a process: people were being arrested, imprisoned, often violently questioned before being, sooner or later, put to death. The study of this repressive system in relation to other communist countries makes one get a fleeting glimpse of specificities: no legal framework to repression, not even a formal one; not any attempt whatsoever to "rehabilitate" the prisoners, though this was typical of the communist prison phenomenon; little concern was shown to prison labour, whereas other regimes massively used it. Above all, the relative death-rate recorded by this system was awesome, compared to other communist experiences. It seems violence was brought to its height by this repressive regim, wich raises a problem for the researcher as to how qualify their acts.