Social reciprocity, empathy, and pretend play are major achievements of toddlerhood, but are impaired or absent in young children with autism. This prospective longitudinal study, which began in infancy, will examine early social and emotional development in high risk toddlers who have an older sibling with an autism diagnosis and compare them to toddlers with a typically developing older sibling at 22, 28, and 34 months. Early social-emotional reciprocity with primary caregivers, the emergence of empathy, the development of pretend play and self-other representation, and the regulation of behavior and negative emotion, all salient developmental tasks of toddlerhood, will be assessed. Cross- sectional and longitudinal analyses will delineate group differences and trajectories of typical and atypical development as a function of diagnosis at 36 months. This work will contribute to early identification and early intervention for children with autism, and also help to differentiate early social markers of autism from those of other developmental delays and typical, but uneven, development.