Over the past decade, the University of California at Los Angeles Post-traumatic Stress Disorder Reaction Index has been one of the most widely used instruments for the assessment of traumatized children and adolescents. This paper reviews its development and modifications that have been made as the diagnostic criteria for post-traumatic stress disorder have evolved. The paper also provides a description of standard methods of administration, procedures for scoring, and psychometric properties. The Reaction Index has been extensively used across a variety of trauma types, age ranges, settings, and cultures. It has been broadly used across the US and around the world after major disasters and catastrophic violence as an integral component of public mental health response and recovery programs. The Reaction Index forms part of a battery that can be efficiently used to conduct needs assessment, surveillance, screening, clinical evaluation, and treatment outcome evaluation after mass casualty events.