The author argues that the Calas Affair is still alive in the collective memory of the French people. His lecture deals with this incident of intolerance and its connection to Voltaire’s famous Traité sur la tolerance [Treatise on tolerance]. The lecture begins with the facts: the death of the young Marc Antoine Calas in the ground floor of the family home in Toulouse (attributed to a possible suicide or unsolved murder) which was unhesitatingly attributed to the father, Jean Calas, a Huguenot cloth merchant. But this is only one of the parameters of the affair. Another concerns the involvement of Voltaire, that led to the Traité sur la tolerance as well as his efforts for the restoration of Jean Calas’s reputation after his death.