In this project, social visual engagement in infants from 2 to 24 months of age will be assessed using eye-tracking technology and innovative quantification of visual attention to study how infants with autism interact with the social world. Experimental data will be collected at 2, 4, 6, 9, 12, 18, and 24 months. A cohort of 235 infants will be enrolled in this project, consisting of 135 infant siblings of children with autism who are at High Risk for developing an ASD (HR-ASD); 50 infants at High Risk for Developmental Delays without familial history of ASD (HR-DD); and 50 children at Low Risk of developmental problems with Typical Development expected (LR-TD expected). By utilizing a longitudinal, prospective study design in at-risk infants, researchers will be able to track the developmental course of altered social engagement in infants subsequently diagnosed with an autism spectrum disorder. The three long-term goals of this research - quantitative diagnostic markers, predictors of outcome, and endophenotypes capable of parsing the heterogeneity of the broader autism spectrum - are also key objectives of the Interagency Autism Coordinating Committee.