Current estimates suggest that one in 150 children have a diagnosis of an autism spectrum disorder (ASD), but there is also striking heterogeneity in social and communication abilities across the spectrum. Understanding the factors that contribute to the emergence and growth of social and communication skills in the early development of children with ASD is critical to understanding this heterogeneity and in the design and targeting of individualized intervention. This project examines the contribution of attention and regulation to individual differences in social and communication skill in the first five years of life, with the goal of identifying predictors of developmental course and potential targets for intervention. Young children with ASD, children with neurotypical development (NT), and younger siblings of children with ASD will be studied, all between 9- and 48-months of age. Overall, this project assesses whether measures of attention and cardiovascular reactivity may be predictors of social-communication growth trajectories and indicators of targets for individualized intervention.