PROJECT SUMMARY/ABSTRACT Adolescence is when many individuals begin to seek increased levels of autonomy from their families and experience pubertal-related brain changes, often leading to increased reward seeking and subsequent risk- taking behaviors. Risky behaviors can lead to injuries, such as traumatic brain injuries (TBIs), which in adolescents and young adults account for a large portion of emergency room visits. TBIs can impact normative brain development and put adolescents at elevated risk for increased mental health symptoms, such as anxiety, depression, and addiction. In fact, peak substance use often occurs during the adolescent period of development, (i.e., alcohol, nicotine, and cannabis) and TBIs may further increase this risk for substance misuse through damage to impulse control regions of the brain, such as the frontal lobe. Personality factors such as impulsivity may also predispose some adolescents to engage in more behaviors that culminate in TBIs and increase risk for poorer health outcomes. However, little is known about the prospective relationships between TBI history, mental health functioning, and substance misuse among individuals as young as 9-14 years of age. Therefore, this project will use data from the Adolescent Brain Cognitive Development (ABCD) Study cohort to examine how mental health symptoms might mediate the association between TBI and substance misuse behaviors and how trait impulsivity might moderate this association. These associations will be examined longitudinally where TBI, mental health symptoms, and substance misuse will be examined at baseline (ages 9-10), year 2 (ages 11-12), and year 4 (ages 13-14) using a cross-lagged panel mediation model with baseline impulsivity as a moderator. The results from this project will inform behavioral challenges, prevention strategies, and functional targets for intervention development for adolescents with history TBI.