This paper is a critical review of the psychology of the criminal, and is of special interest as an indication of the present tendencies of thought in the more progressive section of the Italian school of criminology. At the outset, the author lays stress on the fact that the study of the criminal is changing its direction and is becoming psycho logical rather than morphological. The insistence on the evidence of somatic abnormality, he points out, was useful and, indeed, essential when it was necessary to combat the old views which saw in the criminal a normal individual doing evil of his free will, but it is out of place nowadays when the pathological nature of crime is fully recognised. What is now needed is a more minute study of the psychological mechanism of the criminal. Approaching the question, then, from this side, Angiolella first discusses the relation of crime to insanity, and points out that the affinities of these two conditions have been emphasised of late years as a result of the tendency to abandon the old antithesis between abnormality and disease, and to assign an increasing importance to the rôleof congenital predisposition in insanity. It has, however, to be borne in mind that, even admitting that a common basis of degeneracy underlies certain forms of insanity and certain criminal types, we have still to explain why it is that in one case and not in the other this condition is associated with anti-social tendencies. To meet this difficulty we must assume that an additional factor is operative in the case of the criminal. This factor cannot be, as is sometimes asserted, a special defect of the so-called moral sense; for, since what we so term is no more than the sum of the latest evolved and most complex ideational and emotional associations, we necessarily find it to some extent enfeebled in all the mentally diseased and degenerate, whether they be criminal or not. The further element which is needed to produce the criminal 1st, hen, to be sought rather in the exaggeration and the perversion of his impulses, which the weakness of inhibition allows to issue in anti-social conduct. And this explains the fact that the insane who commit crimes are for the most part those whose insanity is of the degenerate type-paranoiacs, for instance, and epileptics. The insanities of the previously valid brain, the acute psychoses in general, do not lead to gravely criminal acts except in what is to be regarded as a purely accidental way, as may occur in states of hallucinatory confusion. Thus the lunatic who commits a deliberate crime acts, according to this view, under the influence of a temperament which is fundamentally identical with that of the offender whose ill-doing is not associated with any disorder of thought. Passing, then, to the consideration of these perverted impulses which are the essential factor in crime, and discussing more particularly the impulses to homicidal and sexual violence, the author reduces them mainly to exaggerations of individualism and points out that they are fostered by all the influences in social life which exalt brute force over moral force. The combating of these influences must lie at the root of the social prophylaxis of crime, as the prevention of the multiplication of the degenerate must be the aim of its biological prophylaxis.