PROJECT SUMMARY/ABSTRACT Research has shown that the security of a child's attachment to their primary caregiver is crucial to their social relationships and overall well-being. Attachment security is developed through contingent and responsive interactions between the child and caregiver, with physical proximity playing a vital role in building trust. Despite the importance of real-world caregiver–child interactions, previous research has largely focused on laboratory-assessed caregiver quality. To address this gap, we have developed a wearable device called TotTag, which can dynamically and unobtrusively measure real-time physical proximity between device wearers. TotTags are worn in vests by children and in waistbands by caregivers to assess patterns of close contact in children's everyday lives. In this NIH exploratory/developmental research grant, we plan to recruit 100 families with a 12-month-old child from diverse racial, ethnic, and socioeconomic backgrounds. During a home visit, we will assess caregiver sensitivity with up to two caregivers per child. Each caregiver and the infant will then wear TotTags from the child's wake time to bedtime (~12 hours) to assess time in close proximity and the number of caregiver–child "check-ins" (i.e., periods of close contact between periods of separation). Finally, we will conduct the Strange Situation Procedure to assess attachment security with each available caregiver. Aim 1 will explore aspects of caregiver–child proximity in relation to attachment security. Aim 2 will examine the potential additive and interactive contributions of caregiver sensitivity and caregiver– child proximity in relation to attachment security. By obtaining sensitivity and proximity data at the dyad level (e.g., child with mother; child with father) within a family, we aim to gain insight into the specific aspects of each caregiver–child relationship associated with attachment security. Furthermore, this rich intensive longitudinal data of proximity patterns will be made available to allow for exploration of other relevant questions of interest (e.g., associations with family structure; weekend vs. weekday patterns). In addition, we will obtain information regarding the fidelity and acceptability of ecological assessments in families with infants in anticipation of a larger program of research examining the longitudinal development of attachment starting in early infancy. This cross-sectional study, using innovative new technology coupled with home-based caregiver sensitivity assessments and gold-standard laboratory assessments of attachment, will provide an unprecedented picture of infants' daily experiences with their caregivers. Findings from this study will aid in setting the foundation for future work focused on fostering healthy development through secure attachment relationships.