Internalizing disorders are among the most prevalent forms of psychopathology in early childhood with negative consequences that persist across the life course. Although parental depression heightens risk for children's internalizing problems, a significant proportion of at-risk offspring do not develop psychopathology. The factors that render children at risk or confer resilience in the context of parental depression remain poorly understood and in particular, scarce research has explored physiological moderators of the relations between parental depression and children's internalizing. Variations in children's integrated, multisystem stress physiology and parent-child physiological synchrony may distinguish children who are resilient from those who are adversely affected by parental depression. Moreover, stress-sensitive physiological pathways are a promising, but understudied target of treatments that can help elucidate the etiology of children's internalizing disorders and contribute to more effective interventions. This K23 Award builds upon the candidate's prior training and experience to fill critical gaps in the areas of developmental psychopathology and psychobiology, resilience, and intervention research. First, the candidate will examine the longitudinal, transactional relations between parental depressive symptoms and children's internalizing symptoms using in-depth data collected from two unique, extant longitudinal community samples of children that vary in age and exposure to risk. Second, the candidate will test children's multisystem stress reactivity as a moderator of these relations in the same two cohorts. Finally, the candidate will examine the feasibility of the Attachment and Biobehavioral Catch-up (ABC) intervention in a pilot sample of parent-child dyads with elevated depressive and internalizing symptoms, respectively, and assess the preliminary effects of ABC on children's multisystem physiology and parent-child physiological synchrony. Using the NIMH experimental therapeutics approach to intervention development, the candidate will test whether the ABC intervention improves regulation of these physiological factors as potential mechanisms by which to ultimately reduce children's internalizing symptoms. The ABC program is an empirically-supported biobehavioral intervention that is well-suited to attenuate patterns of physiological dysregulation that may underlie relations between parental depression and children's internalizing symptoms, but to date, it has only been delivered to children in foster care and the child welfare system. This novel pilot study will be the first to test the intervention among parents and children with elevated mental health symptoms and will explore its effects on a broader range of physiological outcomes. This research will advance understanding of the dynamic relations among parent depression, child and parent-child stress physiology, and child internalizing early in life, and will contribute to the development of innovative prevention and treatment programs. Through the proposed training and research, the candidate will contribute to cutting-edge research that promotes positive mental health outcomes among at-risk children and families.