Objective: The purpose of this dissertation is two-fold. First, this project examines instability in psychopathic traits from adolescence to adulthood. The second goal is to examine how well psychopathic traits, assessed in adolescence, predict short- and long-term offending. Specific attention is paid to differences in stability and antisocial behavior between youth measured to be high and low in psychopathic traits as well as between psychopathy subtypes. Method: The first study uses various methods to assess personality stability (e.g. rank-order, mean-level, individual-level, ipsative, and predictive stability) to examine the potential for change in psychopathic traits from adolescence to adulthood. The second study addresses how well adolescent assessments of psychopathic traits predict risk for short- and long-term offending and aggression. Both studies examine a sample of former male juvenile offenders (n = 355) who completed a follow-up assessment within prison institutions or in the community (n = 94) six to eight years later. Results: The dissertation produced two key findings. First, the results revealed low-to-moderate stability in psychopathic traits from adolescence to adulthood, but stability estimates were particularly poor for youth that were high in psychopathy. Second, findings indicated that adolescent psychopathic traits performed quite well in identifying enhanced risk for offending and aggression in the short-term, but do not provide meaningful long-term predictive utility (between 6 and 8 years later) for antisocial behavior in adulthood. Measured changes in psychopathic traits over time appeared to be better suited for long-term prediction. Conclusion: Psychopathic traits are generally unstable across the transition to adulthood, particularly for youth high in levels of psychopathy. Characteristics associated with secondary psychopathy may exacerbate changes in psychopathic traits over time. In addition, the limited stability of these traits likely contributed to the poor long-term predictive utility for adolescent features of psychopathy on adult offending. Thus, the use of these assessments in forensic and legal practice to inform decisions regarding denial of treatment, harsher sentencing, and waiver to criminal court may not be appropriate applications of adolescent psychopathy measurement. Multiple assessments of psychopathy over time may also provide a stronger, dynamic picture of risk change over time.