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. The goals are to examine: 1) perceptual precursors of specific social and communicative skills usually affected in autism, namely gaze, affect, and speech processing; 2) syndrome-specific patterns of delays, abnormalities, and strengths based on the assessment of social, cognitive, communicative, exploratory/play, adaptive, and imitation skills collected at each age level; 3) changes over time in the syndrome expression and establish when and in which domains the developmental trajectories of infants with autism begin to diverge from pathways of typical and developmentally delayed infants. This project will: (1) Identify behavioral markers of autism in the first year of life and provide insights in the underlying pathogenic mechanisms of autism; (2) Define sets of diagnostic criteria for autism and PDD-NOS that take under account developmental changes occurring in the first two years of life; (3) Determine the timing and sequence of skill acquisition in autism (4) Identify predictors and mediators of outcome. This research has great potential to inform clinical practice (early diagnosis), treatment (identification of pivotal target areas at various ages), and theory (e.g., elucidation of the underlying mechanisms through identification of primary symptoms and distortions in the sequence of skills acquisition).