Brought to light in 1919 in the immediate aftermath of the war, the Landru case was one of the most resounding criminal cases of the twentieth century. For the first time in history, public opinion was confronted with an unprecedented form of murder. A married man, a father of four, who was in love with a singer whose lover he had been (and who was thus apparently normal in every respect), was to become the killer we still know today who over a four-year period killed 10 women and one young man. Of course, before him, at the end of the nineteenth century, Joseph Vacher, a criminal nicknamed ‘the Ripper of the South-East’, had committed numerous crimes in a state of delusional vagrancy. But he was successively found to be mad, to have recovered, to be not guilty by reason of insanity and finally to be guilty. What is more, his case gave rise to disputes between experts on the question of madness. This was not so with Landru. With his incredible duplicity and his apparent normality, the latter was to introduce the modern enigma of serial killers into French legal history.