For a long time, the Roman Inquisition was considered to be a fearsome institution, infamous for its cruel proceedings and numerous victims. Those precepts came to test with the opening of the Vatican Secret Archives in 2000 which marked a new era in studying of the inquisition. Original case files and manuals regulating the procedure of the Roman Inquisition became accessible on a large scale. Among these is Fundamentum processus, manual composed by the Perugian inquisitor Vincenzo Castrucci in the XVI century. Even though Castrucci was a Dominican friar and had no formal legal education, his manual is the first one to depict the course of the procedure in a stage by stage manner. His sources are his famous predecessors like Eymerich and Peña, but from them Castrucci retained only what was relevant for the daily use of the local inquisitors. The procedure described in the Fundamentum processus shows that the inquisition was a well-regulated criminal procedure with only slight differences from the secular criminal procedure of the early modern period. Many ill-famed characteristics of the inquisition are reaffirmed in Castrucci’s manual, among them the strict secrecy of the procedure, the use of torture s means of extracting evidence, preventive and indefinite incarceration, legal concept of evidence, lack of confrontation etc. However, for each of these drawbacks there inevitably existed a large number of safeguards that were meant to ensure that the procedure was just and the defendant’s dignity observed. E.g. false witnesses were punished on the basis of lex talionis, the suspect had right to defence attorney, the use of torture was meticulously regulated and previsioned in a limited number of cases. Ultimately, it can be concluded that the balance was tilted in favour of the cognizance of truth on the account of the right to defence. Nonetheless, Castrucci’s Fundamentum processus justifies prevailing modern approach to the inquisition which is rather moderate in its criticism and readier than ever to take into account historical background and its contribution to the progress of the criminal procedural law.