PROJECT SUMMARY/ABSTRACT Autism spectrum disorder (ASD) is currently one of the most heritable, heterogeneous, and prevalent childhood psychopathologies and is characterized by social communicative difficulties, as well as restricted, repetitive behaviors. However, many of these communicative and behavioral difficulties may not fully manifest until later in development. My long-term goal is to improve children’s developmental trajectories by identifying early social difficulties within the first two years to better inform behavioral targets within ongoing intervention efforts (e.g., responsiveness; joint attention). Copious research highlights that difficulties in early social exchanges inform later developmental difficulties in language, cognition, and more advanced social communication exchanges. However, the behavioral methods we currently use to assess mean-level social difficulties (e.g., average vocal exchanges) in young children are often limited by inconsistent findings, sample constraints, and large heterogeneity across children/families. This measurement limitation inevitably complicates our interpretations of which social behaviors are (1) indicative of later ASD diagnosis, and (2) which behaviors should be the focus of intervention efforts. Until recently, available methods to accurately and precisely describe dyadic interactions have been limited- and it is widely recognized that dyadic interactions are not fully captured by mean-level statistics (e.g., total frequency of eye contact). Thus, the overall objective of the proposed study is to apply two distinct temporal-based dyadic methodologies (i.e., state space grids; time varying effect modeling) to existing dyadic data in order to sequentially capture the unfolding temporal pattern present within parent-child interactions in elevated-risk samples. The central goal of this study is to improve current measurement recommendations by providing sequential-based representations of early social exchanges during play, in both developmental monitoring (Aim 1) and parent-mediated intervention (Aim 2) contexts. The proposed study is innovative in its application of two temporal-based dyadic methodologies that have never been applied in elevated-risk samples. At its core, ASD is a social disorder and improving our understanding of how these social difficulties unfold over the first two years of life has the potential to significantly impact our current field recommendations for optimizing children’s social development within ongoing identification and intervention contexts.