The talk will present our conceptual model of bio-behavioral synchrony - the coordination of physiological and behavioral responses between attachment partners during social contact - as a theoretical and empirical framework for the study of attachment bonds and the origins of social reciprocity. I will describe how micro-level social behaviors in the gaze, vocal, affective, and touch modalities are dynamically integrated with online physiological processes and hormonal response to create dyad-specific attachments and support children's capacity to become members of the social group, understand complex social signals, and develop social collaboration. Studies across multiple attachments throughout life and following children from infancy to adolescence are presented to show that the extended oxytocin (OT) system provides the neurohormonal substrate for parental, romantic, and filial attachment; that various forms of close relationships are expressed in similar constellations of synchronized behavior and OT increase; and that OT is stable over time within individuals, is mutually-influencing among close partners; is linked with distinct patterns of brain activations and genetic markers; and that mechanisms of cross-generation and inter-couple transmission relate to coordinated social behavior. Longitudinal studies assessing bio-behavioral processes in conditions associated with high risk for social development, including prematurity, maternal post-partum depression, or war-related trauma detail specific alterations to social behavior and neurohormonal systems and highlight specific targets for intervention. Overall, the findings suggest that human affiliation and social reciprocity develop within the matrix of biological attunement and close behavioral synchrony and have conceptual implications for the study of inter-subjectivity and the formulation of a brain-based epistemology as well as translational implications for the integration of OT and behavioral interventions for the treatment of social disorders originating in early childhood.