Antisocial behavior, in other words, rule-breaking behavior, is a major problem in societies all over the world. Because many antisocial behavioral problems start in childhood or adolescence, the study of such behavior problems during this developmental period should contribute to an understanding of the etiology of adult psychopathology. Improved understanding of the etiology of antisocial behavior may contribute to better treatment and prevention. The overall aim of this thesis was to investigate the influence of genetic and environmental factors in the development of antisocial behavior from childhood to emerging adulthood. The data used in this thesis comes from the Twin study of CHild and Adolescent Development (TCHAD), a Swedish population-based study of 1,480 twin pairs born 1985–1986. The twins and their parents have been contacted on four different occasions (8–9 years, 13–14 years, 16–7 years, and 19–20 years) with good to excellent response rates. Multivariate twin methods were applied to investigate the influence of genetic and environmental effects on antisocial behavior from childhood to emerging adulthood. The results suggest that the genetic and environmental etiology of antisocial behavior differs between boys and girls. Heritability was higher in girls, whereas the shared environment was more important in boys. These sex differences remained during the developmental period studied. Antisocial behavior that persists from early adolescence to emerging adulthood has strong familial effect in both boys and girls, with a limited influence of the unique environment. Further, a substantial genetic overlap was found between psychopathic personality traits and antisocial behavior. This genetic overlap could reflect that psychopathic personality has an important role in mediating genetic effects on antisocial behavior. Alternatively, it may indicate a genetic vulnerability to externalizing psychopathology. Finally, socioeconomic status moderated the influence of genetic and environmental factors on antisocial behavior. Genetic influences on antisocial behavior were more important in adolescents in socioeconomically more advantaged environments, whereas the shared environment was higher in adolescents in socioeconomically less advantaged environments. Future research should address the causes of the sex differences in the genetic and environmental etiology of antisocial behavior. Another important question to answer is whether the genetic factor in persistent antisocial behavior is associated with childhood and adulthood psychopathology. A further aspect to explore is if genetic influences associated with psychopathic personality traits are correlated with later antisocial behavior. Knowledge from such studies will provide tools needed to identify effective intervention targets.