Caregiver-child interactions are characterized by interpersonal rhythms of different scales, from nursery rhymes and affective touch to game routines. These rhythms make the environment more predictable for young children and enable interpersonal synchrony and attunement between caregiver and child. By using simultaneous measures of brain activities from caregiver and child, dual-EEG and dual-fNIRS, we can unravel the neural underpinnings of early interactional dynamics and their rhythmicity. I will present our research addressing factors critical to the establishment of caregiver-child synchrony, such as eye contact and interaction quality, especially behavioral reciprocity and contingency. I will also discuss some of the potential functions of interpersonal neural synchrony in early development, from social learning to effective cooperation and communication.