The paper is focused on the relations between criminological theories and fascist and Nazi criminal law systems, and on their historiographical interpretations. The doctrine of the Italian Positivist School, as well as the ideas of other criminologists such as von Liszt, Prins, Jiménez De Asua, contributed to the shift from penal individualism to social defence. Many tenets of penal liberalism were criticised by the rise of new penology and new criminology between the 1870s and the 1920s: the principle of legality and the constitutional safeguards for individual rights were modified and adapted to the need of social protection against the offender’s dangerousness. The key concepts of preventive justice, preventive detention, social responsibility, and measures of security deeply affected the post-Enlightenment criminal justice. Purpose-oriented penal theories, in addition to the systems of special prevention and social defence that were the cornerstones of criminological movements between the 19th and 20th centuries, turned out to be fertile ground in which totalitarianism could easily take root. As has been argued (Vormbaum; Sbriccoli), Nazism and fascism did not break with the tradition of penal liberalism but represented a sort of ‘radicalisation’ and ‘syncretism’ of ideas that were already embedded in legal culture. It would be misleading to trace the cause of totalitarian penal systems to criminological theories. Nonetheless, the dilemmas and contradictions of criminology that arose since its origin gradually undermined the bases of penal liberalism and weakened the fragile equilibrium of the certainty of the law and the individual guarantees on which it rested. The paper will investigate the conundrum of the authoritarian side of criminology by analysing, in particular, the different point of views of Ferri; Dahm and Schaffstein; Radbruch; Hall; Radzinowicz and Jiménez De Asua.