Participants include families with a child who has been diagnosed with an autism spectrum disorder or typical development and a younger sibling 12 months old or younger. The study protocol includes: (1) comprehensive infant evaluations at the University of Washington Autism Center at 6-months, 12-months, 18-months and 24-months of age. (2) Developmental screening for the older sibling. (3) Questionnaires and phone interviews for parents. (4) Random assignment to an assessment and monitoring intervention group or a University of Washington intervention group. All families will receive advice regarding appropriate intervention services. Families who are assigned to the UW intervention will take part in a parent delivered intervention.