Relational aggression-or behavior intended to harm the relationships of its victims-has been the focus of interdisciplinary study across developmental, clinical, personality, and social psychology in the last several decades. One of the primary measures used to assess relational aggression in youth is the Children's Social Behavior Scale (CSBS; Crick & Grotpeter, 1995), but despite its common usage, the construct validity of this measure has not been comprehensively assessed. In the present study, we used a multistage construct validity framework to thoroughly investigate the nature of relational aggression across six community samples totaling 3,102 youth and their caregivers. We used multiple methods to map the reliability, internal or structural validity, and external validity of this scale. Through these analyses, we found that CSBS Relational Aggression demonstrated strong internal consistency, test-retest, and interrater reliability as well as a robust single factor structure and invariance across multiple demographic groups. External validity analyses positioned relational aggression within a theoretically consistent nomological net including psychopathology, personality, and social developmental factors. Contrary to concerns about the validity of self- and parent-reports of relational aggression, both parent- and youth-report forms of the CSBS Relational Aggression scale demonstrated strong reliability and validity. While construct validation has received inadequate attention in the psychological literature to date, through this project, we aimed to demonstrate how this approach may be used to investigate existing measures across psychological research. (PsycInfo Database Record (c) 2021 APA, all rights reserved).