Autism is a complex congenital neurodevelopmental disorder with recurrence risk in younger siblings of affected individuals reaching at least 8%. Despite parental concern arising in children as young as one year old, evidence regarding expression of autism in the first two years of life is very limited. This prospective study will examine development of infant siblings of children with autism contrasted with that of typically developing infants and infants at risk for developmental delays without autistic features. The infants will complete a series of conceptually linked assessment procedures targeting perceptual, social, communicative, cognitive (verbal and nonverbal), motor, and adaptive development at 3, 6, 9,12, 18, and 24 months of age, with confirmatory diagnostic assessment at 36 months. This research could inform early diagnosis and treatment as well as elucidate underlying mechanisms of ASD through identification of primary symptoms and distortions in the sequence of skills acquisition.