*Sadism and Masochism.*--The desire to cause pain to the sexual object and its opposite, the most frequent and most significant of all perversions, was designated in its two forms by v. Krafft-Ebing as sadism or the active form, and masochism or the passive form. Other authors prefer the narrower term algolagnia which emphasizes the pleasure in pain and cruelty, whereas the terms selected by v. Krafft-Ebing place the pleasure secured in all kinds of humility and submission in the foreground. The roots of active algolagnia, sadism, can be readily demonstrable in the normal. The sexuality of most men shows a taint of _aggression_, it is a propensity to subdue, the biological significance of which lies in the necessity of overcoming the resistance of the sexual object by actions other than mere _courting_. Sadism would then correspond to an aggressive component of the sexual impulse which has become independent and exaggerated and has been brought to the foreground by displacement. The conception of sadism fluctuates in the usage of language from a mere active or impetuous attitude towards the sexual object to the exclusive attachment of the gratification to the subjection and maltreatment of the object. Strictly speaking only the last extreme case has a claim to the name of perversion. Similarly, the designation of masochism comprises all passive attitude to the sexual life and to the sexual object; in its most extreme form the gratification is connected with suffering of physical or mental pain at the hands of the sexual object. Masochism as a perversion seems to be still more remote from the normal sexual life by forming a contrast to it; it may be doubted whether it ever appears as a primary form or whether it does not more regularly originate through transformation from sadism. It can often be recognized that the masochism is nothing but a continuation of the sadism turning against one's own person in which the latter at first takes the place of the sexual object. Analysis of extreme cases of masochistic perversions show that there is a coöperation of a large series of factors which exaggerate and fix the original passive sexual attitude (castration complex, conscience). The pain which is here overcome ranks with the loathing and shame which were the resistances opposed to the libido. Sadism and masochism occupy a special place among the perversions, for the contrast of activity and passivity lying at their bases belong to the common traits of the sexual life.