The concept of masochism is used in both descriptive and explanatory ways to cover a wide variety of clinical phenomena. Although the concept has been thought to refer to a ubiquitous, fundamental, and paradoxical phenomenon, recent discussions reveal growing uncertainty about the clinical value of the term. The origins of the problem are traced here to Freud's early reliance on concepts borrowed from Krafft-Ebing's sexology. Freud later emphasized structural and object-relations issues. This shift of emphasis was associated with the use of child behavior rather than perversion as the prototype of mental function.