OBJECTIVE: Aggression is a complication of many psychiatric conditions in youth, but a need remains to measure its specific behaviors. This study evaluated the psychometric and other features of the Retrospective-Modified Overt Aggression Scale (R-MOAS), a 16-item, adult-informant measure for the frequency of verbal, property-related, physical, and self-directed aggressive behaviors. METHOD: Parents of 4,155 youth, aged 5 to 17 years, completed the R-MOAS following referral for behavioral health concerns from general pediatric settings. Analyses examined the following: (1) score distributions, (2) internal consistency, test-retest reliability, and validity, (3) item response theory (IRT) performance, and (4) factor structure. RESULTS: Scores best fit a zero-modified exponential distribution. Self-directed aggressive behavior decreased less with age among female patients. Cronbach α and McDonald ω were high (0.88 and 0.87, respectively), indicating good internal consistency. Test-retest reliability was 0.70. The pattern of correlations with other measures demonstrated convergent and discriminant validity. IRT analyses showed good discrimination covering a range of scores. IRT supports the ordinality of ratings within items but not the scale's traditional approach to weighting item severity. Factor analysis suggested a 2-factor structure. One factor has high loadings from verbal items and milder physical and property-directed aggression ("Eruptive"), and the other factor's loadings drew from self-directed and more destructive behaviors ("Harmful/Distressed"). Measures of affective disturbances made unique contributions to the Harmful/Distressed factor only, whereas the Eruptive factor showed stronger influences of impulsiveness and externalizing behavior. CONCLUSION: The R-MOAS fulfills psychometric criteria for reliability, validity, and IRT performance. It can be a useful component in clinical care and research for the identification, quantification, and outcome monitoring of aggressive behavior in youth. Scoring using item scores is superior to the weighting methods of prior versions, which should be disfavored in youth populations. Factor structure suggests one phenotype that features verbal and relatively minor forms of aggression and another in which self-directed and severe harmful behaviors accompany greater affective disturbance.