Few longitudinal studies have analyzed how violence exposure (e.g. child maltreatment, witnessing community violence) influence both externalizing and Post-Traumatic Stress (PTS) symptoms among children in foster care. Data from three waves of the National Survey of Child and Adolescent Well-Being (1999textendash 2007) (NSCAW; National Data Archive on Child Abuse and Neglect, 2002) were analyzed to investigate the change trajectories of both externalizing and PTS symptomatology among children with a substantiated report of child maltreatment by Child Protective Services (CPS) between October 1999 and December 2000. This study uses data collected at three time points: baseline and approximately 18 (Wave 3) and 36 (Wave 4) months post-baseline. The Child Behavior Checklist (CBCL) scale measured externalizing symptoms and the Post Traumatic Stress Disorder section of a version of the Trauma Symptom Checklist for Children (TSCC) provided the measure of current trauma-related symptoms or distress. Analyses were conducted using a parallel process growth curve model with a sample of n = 280 maltreated youth between the ages of 8 and 15 following home removal. Findings revealed that initial levels of externalizing and PTS symptomatology were both significantly and positively related and co-develop over time. Externalizing symptom severity remained in the borderline range during the first two years in out-of-home care. Both direct and indirect forms of interpersonal violence exposure were associated with initial level of externalizing symptom and PTS symptom severity, respectively. Taken together, our results suggest an underlying process that links early violence exposure to the co-development and cumulative impact of PTS on externalizing behavior above and beyond experiences of maltreatment. We conclude by discussing the key points of intervention that result from a more nuanced understanding of the longitudinal relationship between PTS and externalizing symptoms and the effect of complex trauma on growth in these symptoms over time.