We propose to conduct a prospective, longitudinal study of toddlers at-risk for ASD compared to typically developing toddlers. Children will complete behavioral testing and an overnight Sleep/EEG as well as MRI at either a 12 or 18 month initial. Follow-up visits that include behavioral assessment will occur at 24 and 36 months for all children (and at 18 months of age for the 12-month cohort). The Sleep/EEG and MRI will be repeated at the 36 month final follow-up.