This grant provides support for an NIH Autism Center of Excellence (ACE). This project is a longitudinal study of over 300 infants at risk for autism based on their family history of having older siblings who are diagnosed with ASD. The developmental trajectories of children from 6 to 36 months will be studied, including subgroups of children who manifest varying patterns of symptoms ranging from full autism to the broader autism phenotype to non-affected siblings. Additionally, the children in this study will be followed to age 5 to describe the developmental trajectory of this cohort more fully. This will enable development of tools for earlier diagnosis and better determination of prognosis.