In Belgium, positivism was integrated in the criminal justice system through the influence of the social defense doctrine, as promoted by Adolphe Prins, one of the founding fathers of the International Union of Penal Law. From the end of the nineteenth century, in a context dominated by certain specific figures of dangerousness (habitual and repeat offenders, insane offenders, vagrants and beggars, and juvenile delinquents), the analytical grid of the Italian school appears relevant to fill in the gaps of the neoclassical criminal ideology, considered “too softâ€, abstract and lax. At the same time, positivist ideas were perceived as a threat to democratic criminal law, based on free will and responsibility, legality, and proportionality. The social defense project then resulted in a compromise between free will and determinism, guilt and dangerousness, with a set of dangerousness laws completing—but not replacing—the neoclassical criminal framework at its fringes.