Infancy is the foundational period for learning from adults, and the dynamics of the social environment have long beenproposed as central to childrens development. Here we reveal a novel, naturalistic approach for studying live interactionsbetween infants and adults. Using functional near-infrared spectroscopy (fNIRS), we simultaneously and continuouslymeasured the brains of infants (9-15 months) and an adult while they communicated and played with each other in realtime. We found that time-locked neural synchrony within dyads was significantly greater when they interacted witheach other than with control individuals. In addition, we found that both infant and adult brains continuously trackedthe moment-to-moment fluctuations of mutual gaze and infant emotion with high temporal precision. This investigationmarks a new means of understanding how the brains and behaviors of infants both shape and reflect those of their caregiversduring real-life communication.