Summary Youth with a history of foster care involvement (FCI) have a 200–400% greater chance of reporting a lifetime or past year mental health (MH) problem (Havlicek, Garcia, & Smith, 2013), and are 3.5 times more likely to commit suicide (Katz et al., 2011), than their peers. There are few preventive interventions specifically designed for these youth, and those that exist have generally examined effects on less specific, immediate “problem” behaviors rather than long-term specific MH disorders. The current proposal uses integrative data analysis (IDA) to harmonize data across seven dual-focused caregiver–youth randomized control trials of interventions designed specifically for FCI youth that shared underlying theories of change (yielding an ethnically diverse combined sample of 1,925 youth at baseline and over 6,500 person-by-time assessments that cover multiple periods of youth development from ages 5– 15 years at baseline). This study addresses key questions about both crossover and long-term intervention effects on multiple MH outcomes in FCI youth that no one study has examined in the past or could answer alone. Efficacy of the interventions on both symptoms and diagnoses of five classes of MH outcomes (depression and anxiety, suicidality, thought problems and psychosis, posttraumatic stress disorder, and aggression and violence) will be examined. Increased sample size will also allow for examination of the impact of interventions on low base rate MH outcomes (e.g., suicidality, psychoticism). Using IDA to leverage multiple datasets, the current proposal could positively impact the lives of FCI youth by enabling us to understand if and for how long (Aim 1) preventative interventions implemented early in life may have positive impacts on FCI youth MH, as well as the mechanisms through which (i.e., improved parenting skills and decreased youth emotional and behavioral dysregulation; Aim 2) and for which subpopulations those interventions might be most efficacious (i.e., for females versus males, in the developmental period in which the MH outcome typically manifests [e.g., childhood for violence and aggression, and early to late adolescence for all other MH outcomes], for FCI youth who have experienced more placement changes and received greater intervention dosage, and by ethnic subgroups; Aim 3).