During his papacy (1227-1241), widely acknowledged by scholars as the key moment for the birth of medieval inquisition, Gregory IX wrote more than 300 letters about heresy, antiheretical tasks and inquisition, collected and edited for the first time all togheter here in the second volume. This preliminary work offered the chance to analyse the devolopment of papal antiheretical efforts and to identify the most concerned areas and years of repression. Therefore, a variety of approaches adopted by the pope derived from different political situations and as a consequence it led to a rich and motley lexicon according to single issues and provinces. Furthermore it is also evident the evolution of the role played by members of the Order of the Preachers: during the 1220s they were involved in antiheretical preaching and reconciliation of those found infamous or guilty; later, during the 1240s some friars were committed as inquisitores haereticae pravitatis in southern France.