Increasing interest in “juvenile psychopathy” has been met with scholarly debate about the validity of directly extending the adult construct of psychopathic personality disorder to youth. To inform this debate, this study of 160 serious adolescent offenders compared two alternative, adult-based conceptualizations of juvenile psychopathy: that of the Psychopathy Checklist: Youth Version (PCL:YV) and the self-report Youth Psychopathic traits Inventory (YPI). The results indicate that these two conceptualizations overlap only partially, with the YPI focusing more tightly on core interpersonal and affective features than the PCL:YV. Each conceptualization is reliable and predicts different forms of short-term institutional misbehavior. However, only the YPI possesses a theoretically coherent, inverse association with anxiety. Despite this promise, these conceptualizations of psychopathy are less strongly associated with one another than they are with psychosocial markers of developmental maturity. This raises questions about their divergent validity and ability to identify a disorder that will remain stable during the transition from adolescence into adulthood. Implications for future longitudinal research on the validity, manifestations, and course of juvenile psychopathy are discussed.