We recently demonstrated that a critical infant adaptive social behavior -- looking into the eyes of an engaging caregiver -- obeys a fundamental biological principle of ‘entrainment’: Infant eye-looking entrains (or becomes time-locked) to the rhythmic social cueing of a caregiver during social musical interactions of infant-directed singing. Equally importantly, caregivers structure their own child-directed behavior to enhance this rhythmic cuing and facilitate the delivery and receipt of meaningful social information. These results inform basic mechanisms of typical social development as well as disruptions in social development in children with ASD. A common and lifelong neurodevelopmental disorder, individuals with ASD exhibit impairments in social and communicative functioning that require specialized support. In pilot data for the current proposal, we observe that toddlers with ASD show attenuated, though present, rhythmic entrainment to predictable child-directed singing. The current project builds upon our findings of rhythmic social entrainment during infancy to advance mechanistic understanding of rhythmic entrainment in social development in typically developing toddlers and those with ASD, as well as propose rhythmic entrainment as an active ingredient of music-based interventions for social communication in toddlers with ASD. In the R61, we first quantify the effects of rhythmic entrainment to child- directed singing in toddlers with and without ASD (R61 Aim 1) and examine predictability as a driver of this entrainment (R61 Aim 2). Successfully establishing rhythmic entrainment during predictable social musical engagement (Go/No-Go criteria) will provide strong evidence of a potential fundamental role of rhythm sensitivity in social entrainment. In the R33, we measure the extent to which individual levels of rhythmic entrainment in ASD are mechanistic predictors of response to music-enhanced and standard evidence-based naturalistic developmental behavioral interventions for ASD (R33 Aim 3). Establishing malleability in social rhythm sensitivity is a crucial step for identifying potential mechanisms of change for future investigations of music-based treatments for functional social communication outcomes in ASD. In alignment with RFA-AT-19-001, Promoting Research on Music and Health, this project will facilitate rigorous studies of child health and development and musical interventions. Through examination of the principles of social entrainment afforded by natural social musical interactions, this research has implications for basic mechanisms of disrupted interpersonal synchrony in ASD, while also identifying potential targets of active engagement for the development of music-based interventions.