The chapter begins with a discussion of the classical and positivist traditions in criminology which are concerned with implicit assumptions about individuals and their behaviors and are, in fact, diametrically opposed schools of thought on the nature of human behavior. The emergence of the neoclassical tradition is briefly discussed before the authors turn to a survey of the major biological and psychological theories of crime and deviance. Most of the theories presented are derived from a positivist position that deviance is the outcome of forces beyond the control of the individual. Biological explanations of deviance have focused on physical appearance, genetics and inheritance, and biosocial factors related to how individuals respond to, and in turn shape, their surrounding environment. The implications of biological explanations of deviance for juvenile justice are briefly considered before the authors move on to an examination of the major psychological theories of deviance which tend to focus on treating individuals who have already become deviant rather than on preventing deviance. Psychological explanations include psychoanalytic theories in the tradition of Freud and developmental theories, such as Kohlberg’s model of moral development. Finally, the intersection of personality, mental deficiency, and delinquency is explored. Implications of the psychological explanations of deviance for juvenile justice are considered. Four discussion questions at the end of the chapter focus attention on the classical and positivist schools of thought, biosocial explanations of deviance, methods for explaining "criminal personalities," and the impact of psychological theories of deviance on juvenile justice.