Attention-sharing is noticing what another individual is attending to, and shifting one’s own attention to it because the other is attending to it. This is a distinctive human social behavior that emerges during the first two years of life. Although much descriptive evidence has been reported on the development of attention-sharing (mostly on older infants’ tendency to follow their caregiver’s gaze), we know very little about how attention-sharing emerges in the first half-year of life. We also do not know how caregivers behave with infants to provide the raw data for infants to learn attention-sharing. The central problem is that we lack a parsimonious theory of the emergence of attention-sharing skills. This project, which is based at the University of California San Diego, is designed to find the antecedents of attention-sharing in infants. One part of the project is a longitudinal behavioral study of infants and parents, to find out “what matters” for acquiring the first attention-sharing skills. The other part is a series of computational and robotic models of infant-parent interactions, using data from the longitudinal study based on a new theory of the emergence of early attention-sharing behaviors.