PROJECT SUMMARY/ ABSTRACT Infant development is largely shaped by caregiver-infant interactions, and such interactions are often impaired in families at risk of maltreatment. During healthy caregiver-infant interactions, caregivers and infants co- construct physiological and behavioral exchanges ? known as concordant coregulation ? that are thought to support infant adaptation to stress and serve as an important biomarker of risk. In maltreating families, this coregulation manifests as discordance between caregiver and infant physiology and behavior, thereby increasing vulnerability for poor infant development. While evidence suggests discordance in maltreating mother-infant dyads ? that is, a negative or absence of association of mother-infant coregulation over time ? the extent to which fathers can match their infant?s physiological and behavioral states remains unanswered. Moreover, despite knowledge that fathers are often overrepresented as perpetrators of more severe forms of maltreatment, we lack an advanced understanding of the role of fathers in exacerbating or protecting against maltreatment risk as well as in shaping infant regulatory processes and developmental outcomes. This study will be among the first to examine patterns of mother-infant versus father-infant biobehavioral coregulation among families oversampled for risk. The goal of this study is to advance understanding of the etiology of child maltreatment and infant development in terms of the role of individual and dyadic caregiver-infant regulation and address the following aims: (1) examine patterns of coregulation among mother-infant versus father-infant dyads, and (2) investigate the extent to which patterns of mother-infant versus father-infant coregulation affects maltreatment risk and infant development. Study methods will involve a brief, longitudinal design with self- report, observational, physiological, and behavioral assessments of 60 families from dual-caregiver households with an infant between 6 to 9 months old. This study will have high impact because it will elucidate novel dyadic biobehavioral processes and the role of fathers in affecting maltreatment risk and infant development. Collectively, this work has translational implications for identifying mechanistic pathways to inform maltreatment prevention and intervention programming to promote health and wellness among high-risk caregiver-infant dyads.