Children with Autism Spectrum Disorder (ASD) exhibit numerous impairments in social interaction that typically persist throughout adolescence and adulthood. These deficits severely impede mental and physical development, learning, and behavioral functioning at home and in the community and also make successful treatment difficult. Past research has found that the lack of social competence of children with ASD is comprised of deficits in a number of componential areas including social cognitive and social perceptual processes. Although interacting competently with others relies on cognitive abilities such as making inferences about another's mental state, a less obvious component of social competence lies within social motor processes, the interpersonal coordination of movements during a social interaction. Indeed social psychological research has found that social motor coordination both in the form of imitation and in the lesser known phenomenon of interactional synchrony, is important for maintaining critical aspects of successful human social interaction, including interpersonal responsiveness, social rapport and other-directedness, positive self-other relations, and verbal communication and comprehension. Given the importance of social competence in ASD, the proposed research will evaluate whether social motor coordination can be a marker for social competence in autism across the spectrum of deficits and explore the relationship between motorically-based and cognitively-based conceptions of social competence. Given that social interactions are inherently complex and unfold over time, we propose to evaluate not only traditional cognitive measures of social competence but also the dynamical structure of social coordination across the ASD spectrum by using unique, process-oriented measures of social coordination and analyzing the time series records of the time-dependent unfolding of social coordination during social interaction tasks. We investigate a broad range of questions concerning the social deficits in ASD by using a multi-method design and examining the relationships among measures that capture varied and complex aspects of social interactions. Additionally, we will confront the heterogeneity within the population of children with ASD by including children with a range of language and cognitive abilities in order to get a better sense of how deficits in motor coordination may be influenced by these variables that vary widely within the ASD population. If successful, the research proposed here has important implications for understanding more about the etiology of the social deficits in ASD and will provide a fertile ground for exploring potential new avenues for intervention.