Children involved with the foster care system after experiencing parental neglect or abuse are known to face numerous physical, mental, and developmental challenges. Compounding these concerns, youth with a history of maltreatment experience toxic stress that can extend into adulthood. In light of these health risks, timely and thorough physical and mental health care is critical to their well-being. Health care utilization among children in foster care (i.e., out-of-home care), however, is often inconsistent and fragmented, varying based on factors such as a child's age, gender, race/ethnicity, out-of-home caregiver, and specific health needs. The long-term, predictive effects of these child-level predisposing, enabling, and need characteristics on physical and mental health service use among children in extended foster care was investigated for this thesis research. This was a retrospective longitudinal analysis of data from the second National Survey of Child and Adolescent Well-being. The sample consisted of 451 children who were zero to 17 years old and were in out-of-home care during the 36-month study period. Generalized estimating equations were used to construct population-averaged models while specifying a within-subject working correlation matrix to yield unbiased regression estimates. Younger age and placement with a non-kin caregiver were positively associated with physical health care utilization. The caregiver effect was modified by race/ethnicity; Hispanics residing with non-kin caregivers had the highest adjusted odds of physical health service use. Male gender independently predicted greater mental health care utilization. Older age and clinically significant scores on the Child Behavior Checklist were associated with more mental health service use, but their effects varied by race/ethnicity. White children with scores above the clinical cut-off and older children of Other background had the highest adjusted odds of mental health care utilization. Clinical and non-clinical factors impact longitudinal physical and mental health care utilization among children in foster care. A patient- and family-centered, medical home approach may reduce service barriers by increasing communication and collaboration between caregivers, caseworkers, and healthcare providers. Further, future research with foster youth would benefit from following a standard measure of health need and service use to disentangle causes of healthcare disparities.