The author studies the psychogenesis and assessorial determination of harmfulness. He emphasizes that psychiatric criminal assessment actually must take account, not only the problems concerning the subject of responsibility, but also the harmfulness of the delinquents, in order to fix the harmful ability and the possibility of a delictuous reincidence. This subject belongs to the psychiatric assessorial field, and it is not admissible that the determination of harmfulness were a faculty of the judge alone, and that it abstracts of technical tests. The author stresses the importance of these investigations in the general psychiatric hospital, in order to know the criminal potentiality of the patients interned and to prevent the predelictuous harmfulness. The author studies the social and individual causes of harmfulness, considering especially the disorganization and liberation of instincts and the abnormal personality reactions, under the conception of predisposition and incidence. The general diagnosis of harmfulness requires both the determination of the psychic disease and, chiefly, the exact knowledge of the factors molding the personality; to state the diagnosis, it must be taken into consideration the reasons and circumstances of the crimes, in the integration of the so-called "symptomatic value of the delict". The author discusses several formulae and psychological outlines used in the examination of the personality. He studies the harmfulness presumption in the light of the Brasilian Penal Code, and its relation to psychopathology. The author finally enumerates and studies the legal and clinical forms of harmfulness, under the imperatives of social defense.