Objective: Severe reactive aggression poses a major mental health challenge for many families. A lack of validated instrumentation for assessing young children may present a barrier to more effective clinical assessment and treatment. This scoping review evaluates tools currently used in clinical research to assess aggressive behavior and identifies gaps in the evidence base for their use in children under the age of 12 years. Measures were evaluated through an evidence-based assessment framework to support clinical decision-making.Method: A comprehensive review of registered clinical trials targeting childhood aggression in the United States identified relevant instruments; tools cited in three recent reviews of related constructs were also coded. Measures included were available in English, contained at least three items measuring aggressive behavior, and had at least one validation study in children under 12 years of age. Validation studies were identified through structured queries, and information was extracted from full text review of these studies as well as published manuals.Results: Of 167 candidate measures, 17 met inclusion criteria: three broadband and 14 narrow-band. Compared to commercially-distributed measures, free instruments that were more targeted to assess aggression nevertheless had poorer norms and fewer validation studies in children under 12.Conclusion: Improving instrumentation for assessing reactive aggression would address an urgent clinical need and a gap in current research. More work is needed to validate measures of reactive aggression in children under 12, especially studies that include non-clinical comparison samples. Here we recommend broad and narrow measures for providers to use in clinical care, emphasizing tools with good psychometric properties and no cost barrier.