PROJECT SUMMARY/ABSTRACT Autism spectrum disorder (ASD) and Attention-Deficit/Hyperactivity Disorder (ADHD) are two of the most common neurodevelopmental disorders and affect approximately 1 in 54 and 1 in 11 children in the U.S. respectively with a surprisingly high comorbidity rate of up to 70%. Growing literature suggests that early intervention can ameliorate symptoms and reduce burden of suffering for children affected by these disorders and their families. In order for early intervention to be successful, identification of infants at the highest risk for later challenges is necessary. Social communication deficits, atypical attention, and motor skills challenges have been implicated in the development of both ASD and ADHD. However, little is known about the specificity of these features as unique mechanisms related to the development of ASD and ADHD in infancy. Characterizing the profiles of social communication, attention, and motor skills across infants at an elevated genetic likelihood for ASD, infants at an elevated genetic likelihood for ADHD, and infants at a low genetic likelihood for both disorders can provide important information about disorder-specific symptoms of ASD and ADHD. Understanding transdiagnostic and disorder-specific features of ASD and ADHD will lead to improved screening tools and tailored interventions. Thus, the present study leverages a prospective, longitudinal design to study trajectories of social communication, attention, and motor skills and their relation to ASD and ADHD symptomology throughout the first three years of life. Consistent with NIMH’s Research Domain Criteria (RDoC) approach to investigation of mental disorders, this study will use physiological and behavioral measures to assess these constructs. Further, ASD and ADHD symptoms will be assessed on a dimensional basis. Specific Aim 1 will provide novel information about three phenotypes of ASD and ADHD – communication, attention, motor skills – in infants at genetic risk to provide an early profile of infant skills across three groups. Results from this study will provide information about symptoms associated with genetic risk for ASD and ADHD. Specific Aim 2 will investigate a physiological measure of attention as a predictor of ASD and ADHD symptomology. Notably, this aim will address a gap in current literature concerning the association between attention measured physiologically and symptoms of ADHD. Finally, Specific Aim 3 will explore trajectories of social communication, attention, and motor development as predictors for ASD and ADHD symptomology. These research aims will inform the complex mechanisms implicated in the development of ASD and ADHD symptoms.