In order to more fully understand how individual differences influence adaptation to violence, this study examined the moderating influence of resting heart rate (HR) and HR variability (HRV) between community violence (CV) exposure and child reactive/proactive aggression. Forty 7-13-year-old community children self-reported CV exposure (i.e., victimization, witnessing, or hearing about violence) and were assessed for resting HR and HRV. Parents rated them on reactive/proactive aggression. CV victimization was positively related to proactive aggression only in conditions of low HR, and witnessed CV was positively related to reactive aggression only in conditions of high HRV. Main effects were not found for CV exposure or psychophysiological functioning, suggesting the importance of their interaction. Findings are discussed in terms of HR under-arousal, emotion dysregulation, fearlessness, and behavioral disinhibition as components that can increase aggression in response to violent contexts. Findings support a biosocial basis for childhood aggression and have implications for prevention and treatment. © 2008 Wiley Periodicals, Inc.